assertions.js

/*!
 * chai
 * http://chaijs.com
 * Copyright(c) 2011-2014 Jake Luer <jake@alogicalparadox.com>
 * MIT Licensed
 */

module.exports = function (chai, _) {
  var Assertion = chai.Assertion
    , AssertionError = chai.AssertionError
    , flag = _.flag;

  /**
   * ### Language Chains
   *
   * The following are provided as chainable getters to improve the readability
   * of your assertions.
   *
   * **Chains**
   *
   * - to
   * - be
   * - been
   * - is
   * - that
   * - which
   * - and
   * - has
   * - have
   * - with
   * - at
   * - of
   * - same
   * - but
   * - does
   * - still
   * - also
   *
   * @name language chains
   * @namespace BDD
   * @api public
   */

  [ 'to', 'be', 'been', 'is'
  , 'and', 'has', 'have', 'with'
  , 'that', 'which', 'at', 'of'
  , 'same', 'but', 'does', 'still', "also" ].forEach(function (chain) {
    Assertion.addProperty(chain);
  });

  /**
   * ### .not
   *
   * Negates all assertions that follow in the chain.
   *
   *     expect(function () {}).to.not.throw();
   *     expect({a: 1}).to.not.have.property('b');
   *     expect([1, 2]).to.be.an('array').that.does.not.include(3);
   *
   * Just because you can negate any assertion with `.not` doesn't mean you
   * should. With great power comes great responsibility. It's often best to
   * assert that the one expected output was produced, rather than asserting
   * that one of countless unexpected outputs wasn't produced. See individual
   * assertions for specific guidance.
   *
   *     expect(2).to.equal(2); // Recommended
   *     expect(2).to.not.equal(1); // Not recommended
   *
   * @name not
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('not', function () {
    flag(this, 'negate', true);
  });

  /**
   * ### .deep
   *
   * Causes all `.equal`, `.include`, `.members`, `.keys`, and `.property`
   * assertions that follow in the chain to use deep equality instead of strict
   * (`===`) equality. See the `deep-eql` project page for info on the deep
   * equality algorithm: https://github.com/chaijs/deep-eql.
   *
   *     // Target object deeply (but not strictly) equals `{a: 1}`
   *     expect({a: 1}).to.deep.equal({a: 1});
   *     expect({a: 1}).to.not.equal({a: 1});
   *
   *     // Target array deeply (but not strictly) includes `{a: 1}`
   *     expect([{a: 1}]).to.deep.include({a: 1});
   *     expect([{a: 1}]).to.not.include({a: 1});
   *
   *     // Target object deeply (but not strictly) includes `x: {a: 1}`
   *     expect({x: {a: 1}}).to.deep.include({x: {a: 1}});
   *     expect({x: {a: 1}}).to.not.include({x: {a: 1}});
   *
   *     // Target array deeply (but not strictly) has member `{a: 1}`
   *     expect([{a: 1}]).to.have.deep.members([{a: 1}]);
   *     expect([{a: 1}]).to.not.have.members([{a: 1}]);
   *
   *     // Target set deeply (but not strictly) has key `{a: 1}`
   *     expect(new Set([{a: 1}])).to.have.deep.keys([{a: 1}]);
   *     expect(new Set([{a: 1}])).to.not.have.keys([{a: 1}]);
   *
   *     // Target object deeply (but not strictly) has property `x: {a: 1}`
   *     expect({x: {a: 1}}).to.have.deep.property('x', {a: 1});
   *     expect({x: {a: 1}}).to.not.have.property('x', {a: 1});
   *
   * @name deep
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('deep', function () {
    flag(this, 'deep', true);
  });

  /**
   * ### .nested
   *
   * Enables dot- and bracket-notation in all `.property` and `.include`
   * assertions that follow in the chain.
   *
   *     expect({a: {b: ['x', 'y']}}).to.have.nested.property('a.b[1]');
   *     expect({a: {b: ['x', 'y']}}).to.nested.include({'a.b[1]': 'y'});
   *
   * If `.` or `[]` are part of an actual property name, they can be escaped by
   * adding two backslashes before them.
   *
   *     expect({'.a': {'[b]': 'x'}}).to.have.nested.property('\\.a.\\[b\\]');
   *     expect({'.a': {'[b]': 'x'}}).to.nested.include({'\\.a.\\[b\\]': 'x'});
   *
   * `.nested` cannot be combined with `.own`.
   *
   * @name nested
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('nested', function () {
    flag(this, 'nested', true);
  });

  /**
   * ### .own
   *
   * Causes all `.property` and `.include` assertions that follow in the chain
   * to ignore inherited properties.
   *
   *     Object.prototype.b = 2;
   *
   *     expect({a: 1}).to.have.own.property('a');
   *     expect({a: 1}).to.have.property('b');
   *     expect({a: 1}).to.not.have.own.property('b');
   *
   *     expect({a: 1}).to.own.include({a: 1});
   *     expect({a: 1}).to.include({b: 2}).but.not.own.include({b: 2});
   *
   * `.own` cannot be combined with `.nested`.
   *
   * @name own
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('own', function () {
    flag(this, 'own', true);
  });

  /**
   * ### .ordered
   *
   * Causes all `.members` assertions that follow in the chain to require that
   * members be in the same order.
   *
   *     expect([1, 2]).to.have.ordered.members([1, 2])
   *       .but.not.have.ordered.members([2, 1]);
   *
   * When `.include` and `.ordered` are combined, the ordering begins at the
   * start of both arrays.
   *
   *     expect([1, 2, 3]).to.include.ordered.members([1, 2])
   *       .but.not.include.ordered.members([2, 3]);
   *
   * @name ordered
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('ordered', function () {
    flag(this, 'ordered', true);
  });

  /**
   * ### .any
   *
   * Causes all `.keys` assertions that follow in the chain to only require that
   * the target have at least one of the given keys. This is the opposite of
   * `.all`, which requires that the target have all of the given keys.
   *
   *     expect({a: 1, b: 2}).to.not.have.any.keys('c', 'd');
   *
   * See the `.keys` doc for guidance on when to use `.any` or `.all`.
   *
   * @name any
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('any', function () {
    flag(this, 'any', true);
    flag(this, 'all', false);
  });

  /**
   * ### .all
   *
   * Causes all `.keys` assertions that follow in the chain to require that the
   * target have all of the given keys. This is the opposite of `.any`, which
   * only requires that the target have at least one of the given keys.
   *
   *     expect({a: 1, b: 2}).to.have.all.keys('a', 'b');
   *
   * Note that `.all` is used by default when neither `.all` nor `.any` are
   * added earlier in the chain. However, it's often best to add `.all` anyway
   * because it improves readability.
   *
   * See the `.keys` doc for guidance on when to use `.any` or `.all`.
   *
   * @name all
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('all', function () {
    flag(this, 'all', true);
    flag(this, 'any', false);
  });

  /**
   * ### .a(type[, msg])
   *
   * Asserts that the target's type is equal to the given string `type`. Types
   * are case insensitive. See the `type-detect` project page for info on the
   * type detection algorithm: https://github.com/chaijs/type-detect.
   *
   *     expect('foo').to.be.a('string');
   *     expect({a: 1}).to.be.an('object');
   *     expect(null).to.be.a('null');
   *     expect(undefined).to.be.an('undefined');
   *     expect(new Error).to.be.an('error');
   *     expect(Promise.resolve()).to.be.a('promise');
   *     expect(new Float32Array).to.be.a('float32array');
   *     expect(Symbol()).to.be.a('symbol');
   *
   * `.a` supports objects that have a custom type set via `Symbol.toStringTag`.
   *
   *     var myObj = {
   *       [Symbol.toStringTag]: 'myCustomType'
   *     };
   *
   *     expect(myObj).to.be.a('myCustomType').but.not.an('object');
   *
   * It's often best to use `.a` to check a target's type before making more
   * assertions on the same target. That way, you avoid unexpected behavior from
   * any assertion that does different things based on the target's type.
   *
   *     expect([1, 2, 3]).to.be.an('array').that.includes(2);
   *     expect([]).to.be.an('array').that.is.empty;
   *
   * Add `.not` earlier in the chain to negate `.a`. However, it's often best to
   * assert that the target is the expected type, rather than asserting that it
   * isn't one of many unexpected types.
   *
   *     expect('foo').to.be.a('string'); // Recommended
   *     expect('foo').to.not.be.an('array'); // Not recommended
   *
   * `.a` accepts an optional `msg` argument which is a custom error message to
   * show when the assertion fails. The message can also be given as the second
   * argument to `expect`.
   *
   *     expect(1).to.be.a('string', 'nooo why fail??');
   *     expect(1, 'nooo why fail??').to.be.a('string');
   *
   * `.a` can also be used as a language chain to improve the readability of
   * your assertions.
   *
   *     expect({b: 2}).to.have.a.property('b');
   *
   * The alias `.an` can be used interchangeably with `.a`.
   *
   * @name a
   * @alias an
   * @param {String} type
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function an (type, msg) {
    if (msg) flag(this, 'message', msg);
    type = type.toLowerCase();
    var obj = flag(this, 'object')
      , article = ~[ 'a', 'e', 'i', 'o', 'u' ].indexOf(type.charAt(0)) ? 'an ' : 'a ';

    this.assert(
        type === _.type(obj).toLowerCase()
      , 'expected #{this} to be ' + article + type
      , 'expected #{this} not to be ' + article + type
    );
  }

  Assertion.addChainableMethod('an', an);
  Assertion.addChainableMethod('a', an);

  /**
   * ### .include(val[, msg])
   *
   * When the target is a string, `.include` asserts that the given string `val`
   * is a substring of the target.
   *
   *     expect('foobar').to.include('foo');
   *
   * When the target is an array, `.include` asserts that the given `val` is a
   * member of the target.
   *
   *     expect([1, 2, 3]).to.include(2);
   *
   * When the target is an object, `.include` asserts that the given object
   * `val`'s properties are a subset of the target's properties.
   *
   *     expect({a: 1, b: 2, c: 3}).to.include({a: 1, b: 2});
   *
   * When the target is a Set or WeakSet, `.include` asserts that the given `val` is a
   * member of the target. SameValueZero equality algorithm is used.
   *
   *     expect(new Set([1, 2])).to.include(2);
   *
   * When the target is a Map, `.include` asserts that the given `val` is one of
   * the values of the target. SameValueZero equality algorithm is used.
   *
   *     expect(new Map([['a', 1], ['b', 2]])).to.include(2);
   *
   * Because `.include` does different things based on the target's type, it's
   * important to check the target's type before using `.include`. See the `.a`
   * doc for info on testing a target's type.
   *
   *     expect([1, 2, 3]).to.be.an('array').that.includes(2);
   *
   * By default, strict (`===`) equality is used to compare array members and
   * object properties. Add `.deep` earlier in the chain to use deep equality
   * instead (WeakSet targets are not supported). See the `deep-eql` project
   * page for info on the deep equality algorithm: https://github.com/chaijs/deep-eql.
   *
   *     // Target array deeply (but not strictly) includes `{a: 1}`
   *     expect([{a: 1}]).to.deep.include({a: 1});
   *     expect([{a: 1}]).to.not.include({a: 1});
   *
   *     // Target object deeply (but not strictly) includes `x: {a: 1}`
   *     expect({x: {a: 1}}).to.deep.include({x: {a: 1}});
   *     expect({x: {a: 1}}).to.not.include({x: {a: 1}});
   *
   * By default, all of the target's properties are searched when working with
   * objects. This includes properties that are inherited and/or non-enumerable.
   * Add `.own` earlier in the chain to exclude the target's inherited
   * properties from the search.
   *
   *     Object.prototype.b = 2;
   *
   *     expect({a: 1}).to.own.include({a: 1});
   *     expect({a: 1}).to.include({b: 2}).but.not.own.include({b: 2});
   *
   * Note that a target object is always only searched for `val`'s own
   * enumerable properties.
   *
   * `.deep` and `.own` can be combined.
   *
   *     expect({a: {b: 2}}).to.deep.own.include({a: {b: 2}});
   *
   * Add `.nested` earlier in the chain to enable dot- and bracket-notation when
   * referencing nested properties.
   *
   *     expect({a: {b: ['x', 'y']}}).to.nested.include({'a.b[1]': 'y'});
   *
   * If `.` or `[]` are part of an actual property name, they can be escaped by
   * adding two backslashes before them.
   *
   *     expect({'.a': {'[b]': 2}}).to.nested.include({'\\.a.\\[b\\]': 2});
   *
   * `.deep` and `.nested` can be combined.
   *
   *     expect({a: {b: [{c: 3}]}}).to.deep.nested.include({'a.b[0]': {c: 3}});
   *
   * `.own` and `.nested` cannot be combined.
   *
   * Add `.not` earlier in the chain to negate `.include`.
   *
   *     expect('foobar').to.not.include('taco');
   *     expect([1, 2, 3]).to.not.include(4);
   *
   * However, it's dangerous to negate `.include` when the target is an object.
   * The problem is that it creates uncertain expectations by asserting that the
   * target object doesn't have all of `val`'s key/value pairs but may or may
   * not have some of them. It's often best to identify the exact output that's
   * expected, and then write an assertion that only accepts that exact output.
   *
   * When the target object isn't even expected to have `val`'s keys, it's
   * often best to assert exactly that.
   *
   *     expect({c: 3}).to.not.have.any.keys('a', 'b'); // Recommended
   *     expect({c: 3}).to.not.include({a: 1, b: 2}); // Not recommended
   *
   * When the target object is expected to have `val`'s keys, it's often best to
   * assert that each of the properties has its expected value, rather than
   * asserting that each property doesn't have one of many unexpected values.
   *
   *     expect({a: 3, b: 4}).to.include({a: 3, b: 4}); // Recommended
   *     expect({a: 3, b: 4}).to.not.include({a: 1, b: 2}); // Not recommended
   *
   * `.include` accepts an optional `msg` argument which is a custom error
   * message to show when the assertion fails. The message can also be given as
   * the second argument to `expect`.
   *
   *     expect([1, 2, 3]).to.include(4, 'nooo why fail??');
   *     expect([1, 2, 3], 'nooo why fail??').to.include(4);
   *
   * `.include` can also be used as a language chain, causing all `.members` and
   * `.keys` assertions that follow in the chain to require the target to be a
   * superset of the expected set, rather than an identical set. Note that
   * `.members` ignores duplicates in the subset when `.include` is added.
   *
   *     // Target object's keys are a superset of ['a', 'b'] but not identical
   *     expect({a: 1, b: 2, c: 3}).to.include.all.keys('a', 'b');
   *     expect({a: 1, b: 2, c: 3}).to.not.have.all.keys('a', 'b');
   *
   *     // Target array is a superset of [1, 2] but not identical
   *     expect([1, 2, 3]).to.include.members([1, 2]);
   *     expect([1, 2, 3]).to.not.have.members([1, 2]);
   *
   *     // Duplicates in the subset are ignored
   *     expect([1, 2, 3]).to.include.members([1, 2, 2, 2]);
   *
   * Note that adding `.any` earlier in the chain causes the `.keys` assertion
   * to ignore `.include`.
   *
   *     // Both assertions are identical
   *     expect({a: 1}).to.include.any.keys('a', 'b');
   *     expect({a: 1}).to.have.any.keys('a', 'b');
   *
   * The aliases `.includes`, `.contain`, and `.contains` can be used
   * interchangeably with `.include`.
   *
   * @name include
   * @alias contain
   * @alias includes
   * @alias contains
   * @param {Mixed} val
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function SameValueZero(a, b) {
    return (_.isNaN(a) && _.isNaN(b)) || a === b;
  }

  function includeChainingBehavior () {
    flag(this, 'contains', true);
  }

  function include (val, msg) {
    if (msg) flag(this, 'message', msg);

    var obj = flag(this, 'object')
      , objType = _.type(obj).toLowerCase()
      , flagMsg = flag(this, 'message')
      , negate = flag(this, 'negate')
      , ssfi = flag(this, 'ssfi')
      , isDeep = flag(this, 'deep')
      , descriptor = isDeep ? 'deep ' : ''
      , isEql = isDeep ? flag(this, 'eql') : SameValueZero;

    flagMsg = flagMsg ? flagMsg + ': ' : '';

    var included = false;

    switch (objType) {
      case 'string':
        included = obj.indexOf(val) !== -1;
        break;

      case 'weakset':
        if (isDeep) {
          throw new AssertionError(
            flagMsg + 'unable to use .deep.include with WeakSet',
            undefined,
            ssfi
          );
        }

        included = obj.has(val);
        break;

      case 'map':
        obj.forEach(function (item) {
          included = included || isEql(item, val);
        });
        break;

      case 'set':
        if (isDeep) {
          obj.forEach(function (item) {
            included = included || isEql(item, val);
          });
        } else {
          included = obj.has(val);
        }
        break;

      case 'array':
        if (isDeep) {
          included = obj.some(function (item) {
            return isEql(item, val);
          })
        } else {
          included = obj.indexOf(val) !== -1;
        }
        break;

      default:
        // This block is for asserting a subset of properties in an object.
        // `_.expectTypes` isn't used here because `.include` should work with
        // objects with a custom `@@toStringTag`.
        if (val !== Object(val)) {
          throw new AssertionError(
            flagMsg + 'the given combination of arguments ('
            + objType + ' and '
            + _.type(val).toLowerCase() + ')'
            + ' is invalid for this assertion. '
            + 'You can use an array, a map, an object, a set, a string, '
            + 'or a weakset instead of a '
            + _.type(val).toLowerCase(),
            undefined,
            ssfi
          );
        }

        var props = Object.keys(val)
          , firstErr = null
          , numErrs = 0;

        props.forEach(function (prop) {
          var propAssertion = new Assertion(obj);
          _.transferFlags(this, propAssertion, true);
          flag(propAssertion, 'lockSsfi', true);

          if (!negate || props.length === 1) {
            propAssertion.property(prop, val[prop]);
            return;
          }

          try {
            propAssertion.property(prop, val[prop]);
          } catch (err) {
            if (!_.checkError.compatibleConstructor(err, AssertionError)) {
              throw err;
            }
            if (firstErr === null) firstErr = err;
            numErrs++;
          }
        }, this);

        // When validating .not.include with multiple properties, we only want
        // to throw an assertion error if all of the properties are included,
        // in which case we throw the first property assertion error that we
        // encountered.
        if (negate && props.length > 1 && numErrs === props.length) {
          throw firstErr;
        }
        return;
    }

    // Assert inclusion in collection or substring in a string.
    this.assert(
      included
      , 'expected #{this} to ' + descriptor + 'include ' + _.inspect(val)
      , 'expected #{this} to not ' + descriptor + 'include ' + _.inspect(val));
  }

  Assertion.addChainableMethod('include', include, includeChainingBehavior);
  Assertion.addChainableMethod('contain', include, includeChainingBehavior);
  Assertion.addChainableMethod('contains', include, includeChainingBehavior);
  Assertion.addChainableMethod('includes', include, includeChainingBehavior);

  /**
   * ### .ok
   *
   * Asserts that the target is a truthy value (considered `true` in boolean context).
   * However, it's often best to assert that the target is strictly (`===`) or
   * deeply equal to its expected value.
   *
   *     expect(1).to.equal(1); // Recommended
   *     expect(1).to.be.ok; // Not recommended
   *
   *     expect(true).to.be.true; // Recommended
   *     expect(true).to.be.ok; // Not recommended
   *
   * Add `.not` earlier in the chain to negate `.ok`.
   *
   *     expect(0).to.equal(0); // Recommended
   *     expect(0).to.not.be.ok; // Not recommended
   *
   *     expect(false).to.be.false; // Recommended
   *     expect(false).to.not.be.ok; // Not recommended
   *
   *     expect(null).to.be.null; // Recommended
   *     expect(null).to.not.be.ok; // Not recommended
   *
   *     expect(undefined).to.be.undefined; // Recommended
   *     expect(undefined).to.not.be.ok; // Not recommended
   *
   * A custom error message can be given as the second argument to `expect`.
   *
   *     expect(false, 'nooo why fail??').to.be.ok;
   *
   * @name ok
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('ok', function () {
    this.assert(
        flag(this, 'object')
      , 'expected #{this} to be truthy'
      , 'expected #{this} to be falsy');
  });

  /**
   * ### .true
   *
   * Asserts that the target is strictly (`===`) equal to `true`.
   *
   *     expect(true).to.be.true;
   *
   * Add `.not` earlier in the chain to negate `.true`. However, it's often best
   * to assert that the target is equal to its expected value, rather than not
   * equal to `true`.
   *
   *     expect(false).to.be.false; // Recommended
   *     expect(false).to.not.be.true; // Not recommended
   *
   *     expect(1).to.equal(1); // Recommended
   *     expect(1).to.not.be.true; // Not recommended
   *
   * A custom error message can be given as the second argument to `expect`.
   *
   *     expect(false, 'nooo why fail??').to.be.true;
   *
   * @name true
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('true', function () {
    this.assert(
        true === flag(this, 'object')
      , 'expected #{this} to be true'
      , 'expected #{this} to be false'
      , flag(this, 'negate') ? false : true
    );
  });

  /**
   * ### .false
   *
   * Asserts that the target is strictly (`===`) equal to `false`.
   *
   *     expect(false).to.be.false;
   *
   * Add `.not` earlier in the chain to negate `.false`. However, it's often
   * best to assert that the target is equal to its expected value, rather than
   * not equal to `false`.
   *
   *     expect(true).to.be.true; // Recommended
   *     expect(true).to.not.be.false; // Not recommended
   *
   *     expect(1).to.equal(1); // Recommended
   *     expect(1).to.not.be.false; // Not recommended
   *
   * A custom error message can be given as the second argument to `expect`.
   *
   *     expect(true, 'nooo why fail??').to.be.false;
   *
   * @name false
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('false', function () {
    this.assert(
        false === flag(this, 'object')
      , 'expected #{this} to be false'
      , 'expected #{this} to be true'
      , flag(this, 'negate') ? true : false
    );
  });

  /**
   * ### .null
   *
   * Asserts that the target is strictly (`===`) equal to `null`.
   *
   *     expect(null).to.be.null;
   *
   * Add `.not` earlier in the chain to negate `.null`. However, it's often best
   * to assert that the target is equal to its expected value, rather than not
   * equal to `null`.
   *
   *     expect(1).to.equal(1); // Recommended
   *     expect(1).to.not.be.null; // Not recommended
   *
   * A custom error message can be given as the second argument to `expect`.
   *
   *     expect(42, 'nooo why fail??').to.be.null;
   *
   * @name null
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('null', function () {
    this.assert(
        null === flag(this, 'object')
      , 'expected #{this} to be null'
      , 'expected #{this} not to be null'
    );
  });

  /**
   * ### .undefined
   *
   * Asserts that the target is strictly (`===`) equal to `undefined`.
   *
   *     expect(undefined).to.be.undefined;
   *
   * Add `.not` earlier in the chain to negate `.undefined`. However, it's often
   * best to assert that the target is equal to its expected value, rather than
   * not equal to `undefined`.
   *
   *     expect(1).to.equal(1); // Recommended
   *     expect(1).to.not.be.undefined; // Not recommended
   *
   * A custom error message can be given as the second argument to `expect`.
   *
   *     expect(42, 'nooo why fail??').to.be.undefined;
   *
   * @name undefined
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('undefined', function () {
    this.assert(
        undefined === flag(this, 'object')
      , 'expected #{this} to be undefined'
      , 'expected #{this} not to be undefined'
    );
  });

  /**
   * ### .NaN
   *
   * Asserts that the target is exactly `NaN`.
   *
   *     expect(NaN).to.be.NaN;
   *
   * Add `.not` earlier in the chain to negate `.NaN`. However, it's often best
   * to assert that the target is equal to its expected value, rather than not
   * equal to `NaN`.
   *
   *     expect('foo').to.equal('foo'); // Recommended
   *     expect('foo').to.not.be.NaN; // Not recommended
   *
   * A custom error message can be given as the second argument to `expect`.
   *
   *     expect(42, 'nooo why fail??').to.be.NaN;
   *
   * @name NaN
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('NaN', function () {
    this.assert(
        _.isNaN(flag(this, 'object'))
        , 'expected #{this} to be NaN'
        , 'expected #{this} not to be NaN'
    );
  });

  /**
   * ### .exist
   *
   * Asserts that the target is not strictly (`===`) equal to either `null` or
   * `undefined`. However, it's often best to assert that the target is equal to
   * its expected value.
   *
   *     expect(1).to.equal(1); // Recommended
   *     expect(1).to.exist; // Not recommended
   *
   *     expect(0).to.equal(0); // Recommended
   *     expect(0).to.exist; // Not recommended
   *
   * Add `.not` earlier in the chain to negate `.exist`.
   *
   *     expect(null).to.be.null; // Recommended
   *     expect(null).to.not.exist; // Not recommended
   *
   *     expect(undefined).to.be.undefined; // Recommended
   *     expect(undefined).to.not.exist; // Not recommended
   *
   * A custom error message can be given as the second argument to `expect`.
   *
   *     expect(null, 'nooo why fail??').to.exist;
   *
   * The alias `.exists` can be used interchangeably with `.exist`.
   *
   * @name exist
   * @alias exists
   * @namespace BDD
   * @api public
   */

  function assertExist () {
    var val = flag(this, 'object');
    this.assert(
        val !== null && val !== undefined
      , 'expected #{this} to exist'
      , 'expected #{this} to not exist'
    );
  }

  Assertion.addProperty('exist', assertExist);
  Assertion.addProperty('exists', assertExist);

  /**
   * ### .empty
   *
   * When the target is a string or array, `.empty` asserts that the target's
   * `length` property is strictly (`===`) equal to `0`.
   *
   *     expect([]).to.be.empty;
   *     expect('').to.be.empty;
   *
   * When the target is a map or set, `.empty` asserts that the target's `size`
   * property is strictly equal to `0`.
   *
   *     expect(new Set()).to.be.empty;
   *     expect(new Map()).to.be.empty;
   *
   * When the target is a non-function object, `.empty` asserts that the target
   * doesn't have any own enumerable properties. Properties with Symbol-based
   * keys are excluded from the count.
   *
   *     expect({}).to.be.empty;
   *
   * Because `.empty` does different things based on the target's type, it's
   * important to check the target's type before using `.empty`. See the `.a`
   * doc for info on testing a target's type.
   *
   *     expect([]).to.be.an('array').that.is.empty;
   *
   * Add `.not` earlier in the chain to negate `.empty`. However, it's often
   * best to assert that the target contains its expected number of values,
   * rather than asserting that it's not empty.
   *
   *     expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
   *     expect([1, 2, 3]).to.not.be.empty; // Not recommended
   *
   *     expect(new Set([1, 2, 3])).to.have.property('size', 3); // Recommended
   *     expect(new Set([1, 2, 3])).to.not.be.empty; // Not recommended
   *
   *     expect(Object.keys({a: 1})).to.have.lengthOf(1); // Recommended
   *     expect({a: 1}).to.not.be.empty; // Not recommended
   *
   * A custom error message can be given as the second argument to `expect`.
   *
   *     expect([1, 2, 3], 'nooo why fail??').to.be.empty;
   *
   * @name empty
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('empty', function () {
    var val = flag(this, 'object')
      , ssfi = flag(this, 'ssfi')
      , flagMsg = flag(this, 'message')
      , itemsCount;

    flagMsg = flagMsg ? flagMsg + ': ' : '';

    switch (_.type(val).toLowerCase()) {
      case 'array':
      case 'string':
        itemsCount = val.length;
        break;
      case 'map':
      case 'set':
        itemsCount = val.size;
        break;
      case 'weakmap':
      case 'weakset':
        throw new AssertionError(
          flagMsg + '.empty was passed a weak collection',
          undefined,
          ssfi
        );
      case 'function':
        var msg = flagMsg + '.empty was passed a function ' + _.getName(val);
        throw new AssertionError(msg.trim(), undefined, ssfi);
      default:
        if (val !== Object(val)) {
          throw new AssertionError(
            flagMsg + '.empty was passed non-string primitive ' + _.inspect(val),
            undefined,
            ssfi
          );
        }
        itemsCount = Object.keys(val).length;
    }

    this.assert(
        0 === itemsCount
      , 'expected #{this} to be empty'
      , 'expected #{this} not to be empty'
    );
  });

  /**
   * ### .arguments
   *
   * Asserts that the target is an `arguments` object.
   *
   *     function test () {
   *       expect(arguments).to.be.arguments;
   *     }
   *
   *     test();
   *
   * Add `.not` earlier in the chain to negate `.arguments`. However, it's often
   * best to assert which type the target is expected to be, rather than
   * asserting that it’s not an `arguments` object.
   *
   *     expect('foo').to.be.a('string'); // Recommended
   *     expect('foo').to.not.be.arguments; // Not recommended
   *
   * A custom error message can be given as the second argument to `expect`.
   *
   *     expect({}, 'nooo why fail??').to.be.arguments;
   *
   * The alias `.Arguments` can be used interchangeably with `.arguments`.
   *
   * @name arguments
   * @alias Arguments
   * @namespace BDD
   * @api public
   */

  function checkArguments () {
    var obj = flag(this, 'object')
      , type = _.type(obj);
    this.assert(
        'Arguments' === type
      , 'expected #{this} to be arguments but got ' + type
      , 'expected #{this} to not be arguments'
    );
  }

  Assertion.addProperty('arguments', checkArguments);
  Assertion.addProperty('Arguments', checkArguments);

  /**
   * ### .equal(val[, msg])
   *
   * Asserts that the target is strictly (`===`) equal to the given `val`.
   *
   *     expect(1).to.equal(1);
   *     expect('foo').to.equal('foo');
   *
   * Add `.deep` earlier in the chain to use deep equality instead. See the
   * `deep-eql` project page for info on the deep equality algorithm:
   * https://github.com/chaijs/deep-eql.
   *
   *     // Target object deeply (but not strictly) equals `{a: 1}`
   *     expect({a: 1}).to.deep.equal({a: 1});
   *     expect({a: 1}).to.not.equal({a: 1});
   *
   *     // Target array deeply (but not strictly) equals `[1, 2]`
   *     expect([1, 2]).to.deep.equal([1, 2]);
   *     expect([1, 2]).to.not.equal([1, 2]);
   *
   * Add `.not` earlier in the chain to negate `.equal`. However, it's often
   * best to assert that the target is equal to its expected value, rather than
   * not equal to one of countless unexpected values.
   *
   *     expect(1).to.equal(1); // Recommended
   *     expect(1).to.not.equal(2); // Not recommended
   *
   * `.equal` accepts an optional `msg` argument which is a custom error message
   * to show when the assertion fails. The message can also be given as the
   * second argument to `expect`.
   *
   *     expect(1).to.equal(2, 'nooo why fail??');
   *     expect(1, 'nooo why fail??').to.equal(2);
   *
   * The aliases `.equals` and `eq` can be used interchangeably with `.equal`.
   *
   * @name equal
   * @alias equals
   * @alias eq
   * @param {Mixed} val
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function assertEqual (val, msg) {
    if (msg) flag(this, 'message', msg);
    var obj = flag(this, 'object');
    if (flag(this, 'deep')) {
      var prevLockSsfi = flag(this, 'lockSsfi');
      flag(this, 'lockSsfi', true);
      this.eql(val);
      flag(this, 'lockSsfi', prevLockSsfi);
    } else {
      this.assert(
          val === obj
        , 'expected #{this} to equal #{exp}'
        , 'expected #{this} to not equal #{exp}'
        , val
        , this._obj
        , true
      );
    }
  }

  Assertion.addMethod('equal', assertEqual);
  Assertion.addMethod('equals', assertEqual);
  Assertion.addMethod('eq', assertEqual);

  /**
   * ### .eql(obj[, msg])
   *
   * Asserts that the target is deeply equal to the given `obj`. See the
   * `deep-eql` project page for info on the deep equality algorithm:
   * https://github.com/chaijs/deep-eql.
   *
   *     // Target object is deeply (but not strictly) equal to {a: 1}
   *     expect({a: 1}).to.eql({a: 1}).but.not.equal({a: 1});
   *
   *     // Target array is deeply (but not strictly) equal to [1, 2]
   *     expect([1, 2]).to.eql([1, 2]).but.not.equal([1, 2]);
   *
   * Add `.not` earlier in the chain to negate `.eql`. However, it's often best
   * to assert that the target is deeply equal to its expected value, rather
   * than not deeply equal to one of countless unexpected values.
   *
   *     expect({a: 1}).to.eql({a: 1}); // Recommended
   *     expect({a: 1}).to.not.eql({b: 2}); // Not recommended
   *
   * `.eql` accepts an optional `msg` argument which is a custom error message
   * to show when the assertion fails. The message can also be given as the
   * second argument to `expect`.
   *
   *     expect({a: 1}).to.eql({b: 2}, 'nooo why fail??');
   *     expect({a: 1}, 'nooo why fail??').to.eql({b: 2});
   *
   * The alias `.eqls` can be used interchangeably with `.eql`.
   *
   * The `.deep.equal` assertion is almost identical to `.eql` but with one
   * difference: `.deep.equal` causes deep equality comparisons to also be used
   * for any other assertions that follow in the chain.
   *
   * @name eql
   * @alias eqls
   * @param {Mixed} obj
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function assertEql(obj, msg) {
    if (msg) flag(this, 'message', msg);
    var eql = flag(this, 'eql');
    this.assert(
        eql(obj, flag(this, 'object'))
      , 'expected #{this} to deeply equal #{exp}'
      , 'expected #{this} to not deeply equal #{exp}'
      , obj
      , this._obj
      , true
    );
  }

  Assertion.addMethod('eql', assertEql);
  Assertion.addMethod('eqls', assertEql);

  /**
   * ### .above(n[, msg])
   *
   * Asserts that the target is a number or a date greater than the given number or date `n` respectively.
   * However, it's often best to assert that the target is equal to its expected
   * value.
   *
   *     expect(2).to.equal(2); // Recommended
   *     expect(2).to.be.above(1); // Not recommended
   *
   * Add `.lengthOf` earlier in the chain to assert that the target's `length`
   * or `size` is greater than the given number `n`.
   *
   *     expect('foo').to.have.lengthOf(3); // Recommended
   *     expect('foo').to.have.lengthOf.above(2); // Not recommended
   *
   *     expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
   *     expect([1, 2, 3]).to.have.lengthOf.above(2); // Not recommended
   *
   * Add `.not` earlier in the chain to negate `.above`.
   *
   *     expect(2).to.equal(2); // Recommended
   *     expect(1).to.not.be.above(2); // Not recommended
   *
   * `.above` accepts an optional `msg` argument which is a custom error message
   * to show when the assertion fails. The message can also be given as the
   * second argument to `expect`.
   *
   *     expect(1).to.be.above(2, 'nooo why fail??');
   *     expect(1, 'nooo why fail??').to.be.above(2);
   *
   * The aliases `.gt` and `.greaterThan` can be used interchangeably with
   * `.above`.
   *
   * @name above
   * @alias gt
   * @alias greaterThan
   * @param {Number} n
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function assertAbove (n, msg) {
    if (msg) flag(this, 'message', msg);
    var obj = flag(this, 'object')
      , doLength = flag(this, 'doLength')
      , flagMsg = flag(this, 'message')
      , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
      , ssfi = flag(this, 'ssfi')
      , objType = _.type(obj).toLowerCase()
      , nType = _.type(n).toLowerCase()
      , errorMessage
      , shouldThrow = true;

    if (doLength && objType !== 'map' && objType !== 'set') {
      new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
    }

    if (!doLength && (objType === 'date' && nType !== 'date')) {
      errorMessage = msgPrefix + 'the argument to above must be a date';
    } else if (nType !== 'number' && (doLength || objType === 'number')) {
      errorMessage = msgPrefix + 'the argument to above must be a number';
    } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
      var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
      errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
    } else {
      shouldThrow = false;
    }

    if (shouldThrow) {
      throw new AssertionError(errorMessage, undefined, ssfi);
    }

    if (doLength) {
      var descriptor = 'length'
        , itemsCount;
      if (objType === 'map' || objType === 'set') {
        descriptor = 'size';
        itemsCount = obj.size;
      } else {
        itemsCount = obj.length;
      }
      this.assert(
          itemsCount > n
        , 'expected #{this} to have a ' + descriptor + ' above #{exp} but got #{act}'
        , 'expected #{this} to not have a ' + descriptor + ' above #{exp}'
        , n
        , itemsCount
      );
    } else {
      this.assert(
          obj > n
        , 'expected #{this} to be above #{exp}'
        , 'expected #{this} to be at most #{exp}'
        , n
      );
    }
  }

  Assertion.addMethod('above', assertAbove);
  Assertion.addMethod('gt', assertAbove);
  Assertion.addMethod('greaterThan', assertAbove);

  /**
   * ### .least(n[, msg])
   *
   * Asserts that the target is a number or a date greater than or equal to the given
   * number or date `n` respectively. However, it's often best to assert that the target is equal to
   * its expected value.
   *
   *     expect(2).to.equal(2); // Recommended
   *     expect(2).to.be.at.least(1); // Not recommended
   *     expect(2).to.be.at.least(2); // Not recommended
   *
   * Add `.lengthOf` earlier in the chain to assert that the target's `length`
   * or `size` is greater than or equal to the given number `n`.
   *
   *     expect('foo').to.have.lengthOf(3); // Recommended
   *     expect('foo').to.have.lengthOf.at.least(2); // Not recommended
   *
   *     expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
   *     expect([1, 2, 3]).to.have.lengthOf.at.least(2); // Not recommended
   *
   * Add `.not` earlier in the chain to negate `.least`.
   *
   *     expect(1).to.equal(1); // Recommended
   *     expect(1).to.not.be.at.least(2); // Not recommended
   *
   * `.least` accepts an optional `msg` argument which is a custom error message
   * to show when the assertion fails. The message can also be given as the
   * second argument to `expect`.
   *
   *     expect(1).to.be.at.least(2, 'nooo why fail??');
   *     expect(1, 'nooo why fail??').to.be.at.least(2);
   *
   * The aliases `.gte` and `.greaterThanOrEqual` can be used interchangeably with
   * `.least`.
   *
   * @name least
   * @alias gte
   * @alias greaterThanOrEqual
   * @param {Number} n
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function assertLeast (n, msg) {
    if (msg) flag(this, 'message', msg);
    var obj = flag(this, 'object')
      , doLength = flag(this, 'doLength')
      , flagMsg = flag(this, 'message')
      , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
      , ssfi = flag(this, 'ssfi')
      , objType = _.type(obj).toLowerCase()
      , nType = _.type(n).toLowerCase()
      , errorMessage
      , shouldThrow = true;

    if (doLength && objType !== 'map' && objType !== 'set') {
      new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
    }

    if (!doLength && (objType === 'date' && nType !== 'date')) {
      errorMessage = msgPrefix + 'the argument to least must be a date';
    } else if (nType !== 'number' && (doLength || objType === 'number')) {
      errorMessage = msgPrefix + 'the argument to least must be a number';
    } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
      var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
      errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
    } else {
      shouldThrow = false;
    }

    if (shouldThrow) {
      throw new AssertionError(errorMessage, undefined, ssfi);
    }

    if (doLength) {
      var descriptor = 'length'
        , itemsCount;
      if (objType === 'map' || objType === 'set') {
        descriptor = 'size';
        itemsCount = obj.size;
      } else {
        itemsCount = obj.length;
      }
      this.assert(
          itemsCount >= n
        , 'expected #{this} to have a ' + descriptor + ' at least #{exp} but got #{act}'
        , 'expected #{this} to have a ' + descriptor + ' below #{exp}'
        , n
        , itemsCount
      );
    } else {
      this.assert(
          obj >= n
        , 'expected #{this} to be at least #{exp}'
        , 'expected #{this} to be below #{exp}'
        , n
      );
    }
  }

  Assertion.addMethod('least', assertLeast);
  Assertion.addMethod('gte', assertLeast);
  Assertion.addMethod('greaterThanOrEqual', assertLeast);

  /**
   * ### .below(n[, msg])
   *
   * Asserts that the target is a number or a date less than the given number or date `n` respectively.
   * However, it's often best to assert that the target is equal to its expected
   * value.
   *
   *     expect(1).to.equal(1); // Recommended
   *     expect(1).to.be.below(2); // Not recommended
   *
   * Add `.lengthOf` earlier in the chain to assert that the target's `length`
   * or `size` is less than the given number `n`.
   *
   *     expect('foo').to.have.lengthOf(3); // Recommended
   *     expect('foo').to.have.lengthOf.below(4); // Not recommended
   *
   *     expect([1, 2, 3]).to.have.length(3); // Recommended
   *     expect([1, 2, 3]).to.have.lengthOf.below(4); // Not recommended
   *
   * Add `.not` earlier in the chain to negate `.below`.
   *
   *     expect(2).to.equal(2); // Recommended
   *     expect(2).to.not.be.below(1); // Not recommended
   *
   * `.below` accepts an optional `msg` argument which is a custom error message
   * to show when the assertion fails. The message can also be given as the
   * second argument to `expect`.
   *
   *     expect(2).to.be.below(1, 'nooo why fail??');
   *     expect(2, 'nooo why fail??').to.be.below(1);
   *
   * The aliases `.lt` and `.lessThan` can be used interchangeably with
   * `.below`.
   *
   * @name below
   * @alias lt
   * @alias lessThan
   * @param {Number} n
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function assertBelow (n, msg) {
    if (msg) flag(this, 'message', msg);
    var obj = flag(this, 'object')
      , doLength = flag(this, 'doLength')
      , flagMsg = flag(this, 'message')
      , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
      , ssfi = flag(this, 'ssfi')
      , objType = _.type(obj).toLowerCase()
      , nType = _.type(n).toLowerCase()
      , errorMessage
      , shouldThrow = true;

    if (doLength && objType !== 'map' && objType !== 'set') {
      new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
    }

    if (!doLength && (objType === 'date' && nType !== 'date')) {
      errorMessage = msgPrefix + 'the argument to below must be a date';
    } else if (nType !== 'number' && (doLength || objType === 'number')) {
      errorMessage = msgPrefix + 'the argument to below must be a number';
    } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
      var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
      errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
    } else {
      shouldThrow = false;
    }

    if (shouldThrow) {
      throw new AssertionError(errorMessage, undefined, ssfi);
    }

    if (doLength) {
      var descriptor = 'length'
        , itemsCount;
      if (objType === 'map' || objType === 'set') {
        descriptor = 'size';
        itemsCount = obj.size;
      } else {
        itemsCount = obj.length;
      }
      this.assert(
          itemsCount < n
        , 'expected #{this} to have a ' + descriptor + ' below #{exp} but got #{act}'
        , 'expected #{this} to not have a ' + descriptor + ' below #{exp}'
        , n
        , itemsCount
      );
    } else {
      this.assert(
          obj < n
        , 'expected #{this} to be below #{exp}'
        , 'expected #{this} to be at least #{exp}'
        , n
      );
    }
  }

  Assertion.addMethod('below', assertBelow);
  Assertion.addMethod('lt', assertBelow);
  Assertion.addMethod('lessThan', assertBelow);

  /**
   * ### .most(n[, msg])
   *
   * Asserts that the target is a number or a date less than or equal to the given number
   * or date `n` respectively. However, it's often best to assert that the target is equal to its
   * expected value.
   *
   *     expect(1).to.equal(1); // Recommended
   *     expect(1).to.be.at.most(2); // Not recommended
   *     expect(1).to.be.at.most(1); // Not recommended
   *
   * Add `.lengthOf` earlier in the chain to assert that the target's `length`
   * or `size` is less than or equal to the given number `n`.
   *
   *     expect('foo').to.have.lengthOf(3); // Recommended
   *     expect('foo').to.have.lengthOf.at.most(4); // Not recommended
   *
   *     expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
   *     expect([1, 2, 3]).to.have.lengthOf.at.most(4); // Not recommended
   *
   * Add `.not` earlier in the chain to negate `.most`.
   *
   *     expect(2).to.equal(2); // Recommended
   *     expect(2).to.not.be.at.most(1); // Not recommended
   *
   * `.most` accepts an optional `msg` argument which is a custom error message
   * to show when the assertion fails. The message can also be given as the
   * second argument to `expect`.
   *
   *     expect(2).to.be.at.most(1, 'nooo why fail??');
   *     expect(2, 'nooo why fail??').to.be.at.most(1);
   *
   * The aliases `.lte` and `.lessThanOrEqual` can be used interchangeably with
   * `.most`.
   *
   * @name most
   * @alias lte
   * @alias lessThanOrEqual
   * @param {Number} n
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function assertMost (n, msg) {
    if (msg) flag(this, 'message', msg);
    var obj = flag(this, 'object')
      , doLength = flag(this, 'doLength')
      , flagMsg = flag(this, 'message')
      , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
      , ssfi = flag(this, 'ssfi')
      , objType = _.type(obj).toLowerCase()
      , nType = _.type(n).toLowerCase()
      , errorMessage
      , shouldThrow = true;

    if (doLength && objType !== 'map' && objType !== 'set') {
      new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
    }

    if (!doLength && (objType === 'date' && nType !== 'date')) {
      errorMessage = msgPrefix + 'the argument to most must be a date';
    } else if (nType !== 'number' && (doLength || objType === 'number')) {
      errorMessage = msgPrefix + 'the argument to most must be a number';
    } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
      var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
      errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
    } else {
      shouldThrow = false;
    }

    if (shouldThrow) {
      throw new AssertionError(errorMessage, undefined, ssfi);
    }

    if (doLength) {
      var descriptor = 'length'
        , itemsCount;
      if (objType === 'map' || objType === 'set') {
        descriptor = 'size';
        itemsCount = obj.size;
      } else {
        itemsCount = obj.length;
      }
      this.assert(
          itemsCount <= n
        , 'expected #{this} to have a ' + descriptor + ' at most #{exp} but got #{act}'
        , 'expected #{this} to have a ' + descriptor + ' above #{exp}'
        , n
        , itemsCount
      );
    } else {
      this.assert(
          obj <= n
        , 'expected #{this} to be at most #{exp}'
        , 'expected #{this} to be above #{exp}'
        , n
      );
    }
  }

  Assertion.addMethod('most', assertMost);
  Assertion.addMethod('lte', assertMost);
  Assertion.addMethod('lessThanOrEqual', assertMost);

  /**
   * ### .within(start, finish[, msg])
   *
   * Asserts that the target is a number or a date greater than or equal to the given
   * number or date `start`, and less than or equal to the given number or date `finish` respectively.
   * However, it's often best to assert that the target is equal to its expected
   * value.
   *
   *     expect(2).to.equal(2); // Recommended
   *     expect(2).to.be.within(1, 3); // Not recommended
   *     expect(2).to.be.within(2, 3); // Not recommended
   *     expect(2).to.be.within(1, 2); // Not recommended
   *
   * Add `.lengthOf` earlier in the chain to assert that the target's `length`
   * or `size` is greater than or equal to the given number `start`, and less
   * than or equal to the given number `finish`.
   *
   *     expect('foo').to.have.lengthOf(3); // Recommended
   *     expect('foo').to.have.lengthOf.within(2, 4); // Not recommended
   *
   *     expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
   *     expect([1, 2, 3]).to.have.lengthOf.within(2, 4); // Not recommended
   *
   * Add `.not` earlier in the chain to negate `.within`.
   *
   *     expect(1).to.equal(1); // Recommended
   *     expect(1).to.not.be.within(2, 4); // Not recommended
   *
   * `.within` accepts an optional `msg` argument which is a custom error
   * message to show when the assertion fails. The message can also be given as
   * the second argument to `expect`.
   *
   *     expect(4).to.be.within(1, 3, 'nooo why fail??');
   *     expect(4, 'nooo why fail??').to.be.within(1, 3);
   *
   * @name within
   * @param {Number} start lower bound inclusive
   * @param {Number} finish upper bound inclusive
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  Assertion.addMethod('within', function (start, finish, msg) {
    if (msg) flag(this, 'message', msg);
    var obj = flag(this, 'object')
      , doLength = flag(this, 'doLength')
      , flagMsg = flag(this, 'message')
      , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
      , ssfi = flag(this, 'ssfi')
      , objType = _.type(obj).toLowerCase()
      , startType = _.type(start).toLowerCase()
      , finishType = _.type(finish).toLowerCase()
      , errorMessage
      , shouldThrow = true
      , range = (startType === 'date' && finishType === 'date')
          ? start.toISOString() + '..' + finish.toISOString()
          : start + '..' + finish;

    if (doLength && objType !== 'map' && objType !== 'set') {
      new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
    }

    if (!doLength && (objType === 'date' && (startType !== 'date' || finishType !== 'date'))) {
      errorMessage = msgPrefix + 'the arguments to within must be dates';
    } else if ((startType !== 'number' || finishType !== 'number') && (doLength || objType === 'number')) {
      errorMessage = msgPrefix + 'the arguments to within must be numbers';
    } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
      var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
      errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
    } else {
      shouldThrow = false;
    }

    if (shouldThrow) {
      throw new AssertionError(errorMessage, undefined, ssfi);
    }

    if (doLength) {
      var descriptor = 'length'
        , itemsCount;
      if (objType === 'map' || objType === 'set') {
        descriptor = 'size';
        itemsCount = obj.size;
      } else {
        itemsCount = obj.length;
      }
      this.assert(
          itemsCount >= start && itemsCount <= finish
        , 'expected #{this} to have a ' + descriptor + ' within ' + range
        , 'expected #{this} to not have a ' + descriptor + ' within ' + range
      );
    } else {
      this.assert(
          obj >= start && obj <= finish
        , 'expected #{this} to be within ' + range
        , 'expected #{this} to not be within ' + range
      );
    }
  });

  /**
   * ### .instanceof(constructor[, msg])
   *
   * Asserts that the target is an instance of the given `constructor`.
   *
   *     function Cat () { }
   *
   *     expect(new Cat()).to.be.an.instanceof(Cat);
   *     expect([1, 2]).to.be.an.instanceof(Array);
   *
   * Add `.not` earlier in the chain to negate `.instanceof`.
   *
   *     expect({a: 1}).to.not.be.an.instanceof(Array);
   *
   * `.instanceof` accepts an optional `msg` argument which is a custom error
   * message to show when the assertion fails. The message can also be given as
   * the second argument to `expect`.
   *
   *     expect(1).to.be.an.instanceof(Array, 'nooo why fail??');
   *     expect(1, 'nooo why fail??').to.be.an.instanceof(Array);
   *
   * Due to limitations in ES5, `.instanceof` may not always work as expected
   * when using a transpiler such as Babel or TypeScript. In particular, it may
   * produce unexpected results when subclassing built-in object such as
   * `Array`, `Error`, and `Map`. See your transpiler's docs for details:
   *
   * - ([Babel](https://babeljs.io/docs/usage/caveats/#classes))
   * - ([TypeScript](https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#extending-built-ins-like-error-array-and-map-may-no-longer-work))
   *
   * The alias `.instanceOf` can be used interchangeably with `.instanceof`.
   *
   * @name instanceof
   * @param {Constructor} constructor
   * @param {String} msg _optional_
   * @alias instanceOf
   * @namespace BDD
   * @api public
   */

  function assertInstanceOf (constructor, msg) {
    if (msg) flag(this, 'message', msg);

    var target = flag(this, 'object')
    var ssfi = flag(this, 'ssfi');
    var flagMsg = flag(this, 'message');

    try {
      var isInstanceOf = target instanceof constructor;
    } catch (err) {
      if (err instanceof TypeError) {
        flagMsg = flagMsg ? flagMsg + ': ' : '';
        throw new AssertionError(
          flagMsg + 'The instanceof assertion needs a constructor but '
            + _.type(constructor) + ' was given.',
          undefined,
          ssfi
        );
      }
      throw err;
    }

    var name = _.getName(constructor);
    if (name === null) {
      name = 'an unnamed constructor';
    }

    this.assert(
        isInstanceOf
      , 'expected #{this} to be an instance of ' + name
      , 'expected #{this} to not be an instance of ' + name
    );
  };

  Assertion.addMethod('instanceof', assertInstanceOf);
  Assertion.addMethod('instanceOf', assertInstanceOf);

  /**
   * ### .property(name[, val[, msg]])
   *
   * Asserts that the target has a property with the given key `name`.
   *
   *     expect({a: 1}).to.have.property('a');
   *
   * When `val` is provided, `.property` also asserts that the property's value
   * is equal to the given `val`.
   *
   *     expect({a: 1}).to.have.property('a', 1);
   *
   * By default, strict (`===`) equality is used. Add `.deep` earlier in the
   * chain to use deep equality instead. See the `deep-eql` project page for
   * info on the deep equality algorithm: https://github.com/chaijs/deep-eql.
   *
   *     // Target object deeply (but not strictly) has property `x: {a: 1}`
   *     expect({x: {a: 1}}).to.have.deep.property('x', {a: 1});
   *     expect({x: {a: 1}}).to.not.have.property('x', {a: 1});
   *
   * The target's enumerable and non-enumerable properties are always included
   * in the search. By default, both own and inherited properties are included.
   * Add `.own` earlier in the chain to exclude inherited properties from the
   * search.
   *
   *     Object.prototype.b = 2;
   *
   *     expect({a: 1}).to.have.own.property('a');
   *     expect({a: 1}).to.have.own.property('a', 1);
   *     expect({a: 1}).to.have.property('b');
   *     expect({a: 1}).to.not.have.own.property('b');
   *
   * `.deep` and `.own` can be combined.
   *
   *     expect({x: {a: 1}}).to.have.deep.own.property('x', {a: 1});
   *
   * Add `.nested` earlier in the chain to enable dot- and bracket-notation when
   * referencing nested properties.
   *
   *     expect({a: {b: ['x', 'y']}}).to.have.nested.property('a.b[1]');
   *     expect({a: {b: ['x', 'y']}}).to.have.nested.property('a.b[1]', 'y');
   *
   * If `.` or `[]` are part of an actual property name, they can be escaped by
   * adding two backslashes before them.
   *
   *     expect({'.a': {'[b]': 'x'}}).to.have.nested.property('\\.a.\\[b\\]');
   *
   * `.deep` and `.nested` can be combined.
   *
   *     expect({a: {b: [{c: 3}]}})
   *       .to.have.deep.nested.property('a.b[0]', {c: 3});
   *
   * `.own` and `.nested` cannot be combined.
   *
   * Add `.not` earlier in the chain to negate `.property`.
   *
   *     expect({a: 1}).to.not.have.property('b');
   *
   * However, it's dangerous to negate `.property` when providing `val`. The
   * problem is that it creates uncertain expectations by asserting that the
   * target either doesn't have a property with the given key `name`, or that it
   * does have a property with the given key `name` but its value isn't equal to
   * the given `val`. It's often best to identify the exact output that's
   * expected, and then write an assertion that only accepts that exact output.
   *
   * When the target isn't expected to have a property with the given key
   * `name`, it's often best to assert exactly that.
   *
   *     expect({b: 2}).to.not.have.property('a'); // Recommended
   *     expect({b: 2}).to.not.have.property('a', 1); // Not recommended
   *
   * When the target is expected to have a property with the given key `name`,
   * it's often best to assert that the property has its expected value, rather
   * than asserting that it doesn't have one of many unexpected values.
   *
   *     expect({a: 3}).to.have.property('a', 3); // Recommended
   *     expect({a: 3}).to.not.have.property('a', 1); // Not recommended
   *
   * `.property` changes the target of any assertions that follow in the chain
   * to be the value of the property from the original target object.
   *
   *     expect({a: 1}).to.have.property('a').that.is.a('number');
   *
   * `.property` accepts an optional `msg` argument which is a custom error
   * message to show when the assertion fails. The message can also be given as
   * the second argument to `expect`. When not providing `val`, only use the
   * second form.
   *
   *     // Recommended
   *     expect({a: 1}).to.have.property('a', 2, 'nooo why fail??');
   *     expect({a: 1}, 'nooo why fail??').to.have.property('a', 2);
   *     expect({a: 1}, 'nooo why fail??').to.have.property('b');
   *
   *     // Not recommended
   *     expect({a: 1}).to.have.property('b', undefined, 'nooo why fail??');
   *
   * The above assertion isn't the same thing as not providing `val`. Instead,
   * it's asserting that the target object has a `b` property that's equal to
   * `undefined`.
   *
   * The assertions `.ownProperty` and `.haveOwnProperty` can be used
   * interchangeably with `.own.property`.
   *
   * @name property
   * @param {String} name
   * @param {Mixed} val (optional)
   * @param {String} msg _optional_
   * @returns value of property for chaining
   * @namespace BDD
   * @api public
   */

  function assertProperty (name, val, msg) {
    if (msg) flag(this, 'message', msg);

    var isNested = flag(this, 'nested')
      , isOwn = flag(this, 'own')
      , flagMsg = flag(this, 'message')
      , obj = flag(this, 'object')
      , ssfi = flag(this, 'ssfi')
      , nameType = typeof name;

    flagMsg = flagMsg ? flagMsg + ': ' : '';

    if (isNested) {
      if (nameType !== 'string') {
        throw new AssertionError(
          flagMsg + 'the argument to property must be a string when using nested syntax',
          undefined,
          ssfi
        );
      }
    } else {
      if (nameType !== 'string' && nameType !== 'number' && nameType !== 'symbol') {
        throw new AssertionError(
          flagMsg + 'the argument to property must be a string, number, or symbol',
          undefined,
          ssfi
        );
      }
    }

    if (isNested && isOwn) {
      throw new AssertionError(
        flagMsg + 'The "nested" and "own" flags cannot be combined.',
        undefined,
        ssfi
      );
    }

    if (obj === null || obj === undefined) {
      throw new AssertionError(
        flagMsg + 'Target cannot be null or undefined.',
        undefined,
        ssfi
      );
    }

    var isDeep = flag(this, 'deep')
      , negate = flag(this, 'negate')
      , pathInfo = isNested ? _.getPathInfo(obj, name) : null
      , value = isNested ? pathInfo.value : obj[name]
      , isEql = isDeep ? flag(this, 'eql') : (val1, val2) => val1 === val2;;

    var descriptor = '';
    if (isDeep) descriptor += 'deep ';
    if (isOwn) descriptor += 'own ';
    if (isNested) descriptor += 'nested ';
    descriptor += 'property ';

    var hasProperty;
    if (isOwn) hasProperty = Object.prototype.hasOwnProperty.call(obj, name);
    else if (isNested) hasProperty = pathInfo.exists;
    else hasProperty = _.hasProperty(obj, name);

    // When performing a negated assertion for both name and val, merely having
    // a property with the given name isn't enough to cause the assertion to
    // fail. It must both have a property with the given name, and the value of
    // that property must equal the given val. Therefore, skip this assertion in
    // favor of the next.
    if (!negate || arguments.length === 1) {
      this.assert(
          hasProperty
        , 'expected #{this} to have ' + descriptor + _.inspect(name)
        , 'expected #{this} to not have ' + descriptor + _.inspect(name));
    }

    if (arguments.length > 1) {
      this.assert(
          hasProperty && isEql(val, value)
        , 'expected #{this} to have ' + descriptor + _.inspect(name) + ' of #{exp}, but got #{act}'
        , 'expected #{this} to not have ' + descriptor + _.inspect(name) + ' of #{act}'
        , val
        , value
      );
    }

    flag(this, 'object', value);
  }

  Assertion.addMethod('property', assertProperty);

  function assertOwnProperty (name, value, msg) {
    flag(this, 'own', true);
    assertProperty.apply(this, arguments);
  }

  Assertion.addMethod('ownProperty', assertOwnProperty);
  Assertion.addMethod('haveOwnProperty', assertOwnProperty);

  /**
   * ### .ownPropertyDescriptor(name[, descriptor[, msg]])
   *
   * Asserts that the target has its own property descriptor with the given key
   * `name`. Enumerable and non-enumerable properties are included in the
   * search.
   *
   *     expect({a: 1}).to.have.ownPropertyDescriptor('a');
   *
   * When `descriptor` is provided, `.ownPropertyDescriptor` also asserts that
   * the property's descriptor is deeply equal to the given `descriptor`. See
   * the `deep-eql` project page for info on the deep equality algorithm:
   * https://github.com/chaijs/deep-eql.
   *
   *     expect({a: 1}).to.have.ownPropertyDescriptor('a', {
   *       configurable: true,
   *       enumerable: true,
   *       writable: true,
   *       value: 1,
   *     });
   *
   * Add `.not` earlier in the chain to negate `.ownPropertyDescriptor`.
   *
   *     expect({a: 1}).to.not.have.ownPropertyDescriptor('b');
   *
   * However, it's dangerous to negate `.ownPropertyDescriptor` when providing
   * a `descriptor`. The problem is that it creates uncertain expectations by
   * asserting that the target either doesn't have a property descriptor with
   * the given key `name`, or that it does have a property descriptor with the
   * given key `name` but it’s not deeply equal to the given `descriptor`. It's
   * often best to identify the exact output that's expected, and then write an
   * assertion that only accepts that exact output.
   *
   * When the target isn't expected to have a property descriptor with the given
   * key `name`, it's often best to assert exactly that.
   *
   *     // Recommended
   *     expect({b: 2}).to.not.have.ownPropertyDescriptor('a');
   *
   *     // Not recommended
   *     expect({b: 2}).to.not.have.ownPropertyDescriptor('a', {
   *       configurable: true,
   *       enumerable: true,
   *       writable: true,
   *       value: 1,
   *     });
   *
   * When the target is expected to have a property descriptor with the given
   * key `name`, it's often best to assert that the property has its expected
   * descriptor, rather than asserting that it doesn't have one of many
   * unexpected descriptors.
   *
   *     // Recommended
   *     expect({a: 3}).to.have.ownPropertyDescriptor('a', {
   *       configurable: true,
   *       enumerable: true,
   *       writable: true,
   *       value: 3,
   *     });
   *
   *     // Not recommended
   *     expect({a: 3}).to.not.have.ownPropertyDescriptor('a', {
   *       configurable: true,
   *       enumerable: true,
   *       writable: true,
   *       value: 1,
   *     });
   *
   * `.ownPropertyDescriptor` changes the target of any assertions that follow
   * in the chain to be the value of the property descriptor from the original
   * target object.
   *
   *     expect({a: 1}).to.have.ownPropertyDescriptor('a')
   *       .that.has.property('enumerable', true);
   *
   * `.ownPropertyDescriptor` accepts an optional `msg` argument which is a
   * custom error message to show when the assertion fails. The message can also
   * be given as the second argument to `expect`. When not providing
   * `descriptor`, only use the second form.
   *
   *     // Recommended
   *     expect({a: 1}).to.have.ownPropertyDescriptor('a', {
   *       configurable: true,
   *       enumerable: true,
   *       writable: true,
   *       value: 2,
   *     }, 'nooo why fail??');
   *
   *     // Recommended
   *     expect({a: 1}, 'nooo why fail??').to.have.ownPropertyDescriptor('a', {
   *       configurable: true,
   *       enumerable: true,
   *       writable: true,
   *       value: 2,
   *     });
   *
   *     // Recommended
   *     expect({a: 1}, 'nooo why fail??').to.have.ownPropertyDescriptor('b');
   *
   *     // Not recommended
   *     expect({a: 1})
   *       .to.have.ownPropertyDescriptor('b', undefined, 'nooo why fail??');
   *
   * The above assertion isn't the same thing as not providing `descriptor`.
   * Instead, it's asserting that the target object has a `b` property
   * descriptor that's deeply equal to `undefined`.
   *
   * The alias `.haveOwnPropertyDescriptor` can be used interchangeably with
   * `.ownPropertyDescriptor`.
   *
   * @name ownPropertyDescriptor
   * @alias haveOwnPropertyDescriptor
   * @param {String} name
   * @param {Object} descriptor _optional_
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function assertOwnPropertyDescriptor (name, descriptor, msg) {
    if (typeof descriptor === 'string') {
      msg = descriptor;
      descriptor = null;
    }
    if (msg) flag(this, 'message', msg);
    var obj = flag(this, 'object');
    var actualDescriptor = Object.getOwnPropertyDescriptor(Object(obj), name);
    var eql = flag(this, 'eql');
    if (actualDescriptor && descriptor) {
      this.assert(
          eql(descriptor, actualDescriptor)
        , 'expected the own property descriptor for ' + _.inspect(name) + ' on #{this} to match ' + _.inspect(descriptor) + ', got ' + _.inspect(actualDescriptor)
        , 'expected the own property descriptor for ' + _.inspect(name) + ' on #{this} to not match ' + _.inspect(descriptor)
        , descriptor
        , actualDescriptor
        , true
      );
    } else {
      this.assert(
          actualDescriptor
        , 'expected #{this} to have an own property descriptor for ' + _.inspect(name)
        , 'expected #{this} to not have an own property descriptor for ' + _.inspect(name)
      );
    }
    flag(this, 'object', actualDescriptor);
  }

  Assertion.addMethod('ownPropertyDescriptor', assertOwnPropertyDescriptor);
  Assertion.addMethod('haveOwnPropertyDescriptor', assertOwnPropertyDescriptor);

  /**
   * ### .lengthOf(n[, msg])
   *
   * Asserts that the target's `length` or `size` is equal to the given number
   * `n`.
   *
   *     expect([1, 2, 3]).to.have.lengthOf(3);
   *     expect('foo').to.have.lengthOf(3);
   *     expect(new Set([1, 2, 3])).to.have.lengthOf(3);
   *     expect(new Map([['a', 1], ['b', 2], ['c', 3]])).to.have.lengthOf(3);
   *
   * Add `.not` earlier in the chain to negate `.lengthOf`. However, it's often
   * best to assert that the target's `length` property is equal to its expected
   * value, rather than not equal to one of many unexpected values.
   *
   *     expect('foo').to.have.lengthOf(3); // Recommended
   *     expect('foo').to.not.have.lengthOf(4); // Not recommended
   *
   * `.lengthOf` accepts an optional `msg` argument which is a custom error
   * message to show when the assertion fails. The message can also be given as
   * the second argument to `expect`.
   *
   *     expect([1, 2, 3]).to.have.lengthOf(2, 'nooo why fail??');
   *     expect([1, 2, 3], 'nooo why fail??').to.have.lengthOf(2);
   *
   * `.lengthOf` can also be used as a language chain, causing all `.above`,
   * `.below`, `.least`, `.most`, and `.within` assertions that follow in the
   * chain to use the target's `length` property as the target. However, it's
   * often best to assert that the target's `length` property is equal to its
   * expected length, rather than asserting that its `length` property falls
   * within some range of values.
   *
   *     // Recommended
   *     expect([1, 2, 3]).to.have.lengthOf(3);
   *
   *     // Not recommended
   *     expect([1, 2, 3]).to.have.lengthOf.above(2);
   *     expect([1, 2, 3]).to.have.lengthOf.below(4);
   *     expect([1, 2, 3]).to.have.lengthOf.at.least(3);
   *     expect([1, 2, 3]).to.have.lengthOf.at.most(3);
   *     expect([1, 2, 3]).to.have.lengthOf.within(2,4);
   *
   * Due to a compatibility issue, the alias `.length` can't be chained directly
   * off of an uninvoked method such as `.a`. Therefore, `.length` can't be used
   * interchangeably with `.lengthOf` in every situation. It's recommended to
   * always use `.lengthOf` instead of `.length`.
   *
   *     expect([1, 2, 3]).to.have.a.length(3); // incompatible; throws error
   *     expect([1, 2, 3]).to.have.a.lengthOf(3);  // passes as expected
   *
   * @name lengthOf
   * @alias length
   * @param {Number} n
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function assertLengthChain () {
    flag(this, 'doLength', true);
  }

  function assertLength (n, msg) {
    if (msg) flag(this, 'message', msg);
    var obj = flag(this, 'object')
      , objType = _.type(obj).toLowerCase()
      , flagMsg = flag(this, 'message')
      , ssfi = flag(this, 'ssfi')
      , descriptor = 'length'
      , itemsCount;

    switch (objType) {
      case 'map':
      case 'set':
        descriptor = 'size';
        itemsCount = obj.size;
        break;
      default:
        new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
        itemsCount = obj.length;
    }

    this.assert(
        itemsCount == n
      , 'expected #{this} to have a ' + descriptor + ' of #{exp} but got #{act}'
      , 'expected #{this} to not have a ' + descriptor + ' of #{act}'
      , n
      , itemsCount
    );
  }

  Assertion.addChainableMethod('length', assertLength, assertLengthChain);
  Assertion.addChainableMethod('lengthOf', assertLength, assertLengthChain);

  /**
   * ### .match(re[, msg])
   *
   * Asserts that the target matches the given regular expression `re`.
   *
   *     expect('foobar').to.match(/^foo/);
   *
   * Add `.not` earlier in the chain to negate `.match`.
   *
   *     expect('foobar').to.not.match(/taco/);
   *
   * `.match` accepts an optional `msg` argument which is a custom error message
   * to show when the assertion fails. The message can also be given as the
   * second argument to `expect`.
   *
   *     expect('foobar').to.match(/taco/, 'nooo why fail??');
   *     expect('foobar', 'nooo why fail??').to.match(/taco/);
   *
   * The alias `.matches` can be used interchangeably with `.match`.
   *
   * @name match
   * @alias matches
   * @param {RegExp} re
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */
  function assertMatch(re, msg) {
    if (msg) flag(this, 'message', msg);
    var obj = flag(this, 'object');
    this.assert(
        re.exec(obj)
      , 'expected #{this} to match ' + re
      , 'expected #{this} not to match ' + re
    );
  }

  Assertion.addMethod('match', assertMatch);
  Assertion.addMethod('matches', assertMatch);

  /**
   * ### .string(str[, msg])
   *
   * Asserts that the target string contains the given substring `str`.
   *
   *     expect('foobar').to.have.string('bar');
   *
   * Add `.not` earlier in the chain to negate `.string`.
   *
   *     expect('foobar').to.not.have.string('taco');
   *
   * `.string` accepts an optional `msg` argument which is a custom error
   * message to show when the assertion fails. The message can also be given as
   * the second argument to `expect`.
   *
   *     expect('foobar').to.have.string('taco', 'nooo why fail??');
   *     expect('foobar', 'nooo why fail??').to.have.string('taco');
   *
   * @name string
   * @param {String} str
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  Assertion.addMethod('string', function (str, msg) {
    if (msg) flag(this, 'message', msg);
    var obj = flag(this, 'object')
      , flagMsg = flag(this, 'message')
      , ssfi = flag(this, 'ssfi');
    new Assertion(obj, flagMsg, ssfi, true).is.a('string');

    this.assert(
        ~obj.indexOf(str)
      , 'expected #{this} to contain ' + _.inspect(str)
      , 'expected #{this} to not contain ' + _.inspect(str)
    );
  });

  /**
   * ### .keys(key1[, key2[, ...]])
   *
   * Asserts that the target object, array, map, or set has the given keys. Only
   * the target's own inherited properties are included in the search.
   *
   * When the target is an object or array, keys can be provided as one or more
   * string arguments, a single array argument, or a single object argument. In
   * the latter case, only the keys in the given object matter; the values are
   * ignored.
   *
   *     expect({a: 1, b: 2}).to.have.all.keys('a', 'b');
   *     expect(['x', 'y']).to.have.all.keys(0, 1);
   *
   *     expect({a: 1, b: 2}).to.have.all.keys(['a', 'b']);
   *     expect(['x', 'y']).to.have.all.keys([0, 1]);
   *
   *     expect({a: 1, b: 2}).to.have.all.keys({a: 4, b: 5}); // ignore 4 and 5
   *     expect(['x', 'y']).to.have.all.keys({0: 4, 1: 5}); // ignore 4 and 5
   *
   * When the target is a map or set, each key must be provided as a separate
   * argument.
   *
   *     expect(new Map([['a', 1], ['b', 2]])).to.have.all.keys('a', 'b');
   *     expect(new Set(['a', 'b'])).to.have.all.keys('a', 'b');
   *
   * Because `.keys` does different things based on the target's type, it's
   * important to check the target's type before using `.keys`. See the `.a` doc
   * for info on testing a target's type.
   *
   *     expect({a: 1, b: 2}).to.be.an('object').that.has.all.keys('a', 'b');
   *
   * By default, strict (`===`) equality is used to compare keys of maps and
   * sets. Add `.deep` earlier in the chain to use deep equality instead. See
   * the `deep-eql` project page for info on the deep equality algorithm:
   * https://github.com/chaijs/deep-eql.
   *
   *     // Target set deeply (but not strictly) has key `{a: 1}`
   *     expect(new Set([{a: 1}])).to.have.all.deep.keys([{a: 1}]);
   *     expect(new Set([{a: 1}])).to.not.have.all.keys([{a: 1}]);
   *
   * By default, the target must have all of the given keys and no more. Add
   * `.any` earlier in the chain to only require that the target have at least
   * one of the given keys. Also, add `.not` earlier in the chain to negate
   * `.keys`. It's often best to add `.any` when negating `.keys`, and to use
   * `.all` when asserting `.keys` without negation.
   *
   * When negating `.keys`, `.any` is preferred because `.not.any.keys` asserts
   * exactly what's expected of the output, whereas `.not.all.keys` creates
   * uncertain expectations.
   *
   *     // Recommended; asserts that target doesn't have any of the given keys
   *     expect({a: 1, b: 2}).to.not.have.any.keys('c', 'd');
   *
   *     // Not recommended; asserts that target doesn't have all of the given
   *     // keys but may or may not have some of them
   *     expect({a: 1, b: 2}).to.not.have.all.keys('c', 'd');
   *
   * When asserting `.keys` without negation, `.all` is preferred because
   * `.all.keys` asserts exactly what's expected of the output, whereas
   * `.any.keys` creates uncertain expectations.
   *
   *     // Recommended; asserts that target has all the given keys
   *     expect({a: 1, b: 2}).to.have.all.keys('a', 'b');
   *
   *     // Not recommended; asserts that target has at least one of the given
   *     // keys but may or may not have more of them
   *     expect({a: 1, b: 2}).to.have.any.keys('a', 'b');
   *
   * Note that `.all` is used by default when neither `.all` nor `.any` appear
   * earlier in the chain. However, it's often best to add `.all` anyway because
   * it improves readability.
   *
   *     // Both assertions are identical
   *     expect({a: 1, b: 2}).to.have.all.keys('a', 'b'); // Recommended
   *     expect({a: 1, b: 2}).to.have.keys('a', 'b'); // Not recommended
   *
   * Add `.include` earlier in the chain to require that the target's keys be a
   * superset of the expected keys, rather than identical sets.
   *
   *     // Target object's keys are a superset of ['a', 'b'] but not identical
   *     expect({a: 1, b: 2, c: 3}).to.include.all.keys('a', 'b');
   *     expect({a: 1, b: 2, c: 3}).to.not.have.all.keys('a', 'b');
   *
   * However, if `.any` and `.include` are combined, only the `.any` takes
   * effect. The `.include` is ignored in this case.
   *
   *     // Both assertions are identical
   *     expect({a: 1}).to.have.any.keys('a', 'b');
   *     expect({a: 1}).to.include.any.keys('a', 'b');
   *
   * A custom error message can be given as the second argument to `expect`.
   *
   *     expect({a: 1}, 'nooo why fail??').to.have.key('b');
   *
   * The alias `.key` can be used interchangeably with `.keys`.
   *
   * @name keys
   * @alias key
   * @param {...String|Array|Object} keys
   * @namespace BDD
   * @api public
   */

  function assertKeys (keys) {
    var obj = flag(this, 'object')
      , objType = _.type(obj)
      , keysType = _.type(keys)
      , ssfi = flag(this, 'ssfi')
      , isDeep = flag(this, 'deep')
      , str
      , deepStr = ''
      , actual
      , ok = true
      , flagMsg = flag(this, 'message');

    flagMsg = flagMsg ? flagMsg + ': ' : '';
    var mixedArgsMsg = flagMsg + 'when testing keys against an object or an array you must give a single Array|Object|String argument or multiple String arguments';

    if (objType === 'Map' || objType === 'Set') {
      deepStr = isDeep ? 'deeply ' : '';
      actual = [];

      // Map and Set '.keys' aren't supported in IE 11. Therefore, use .forEach.
      obj.forEach(function (val, key) { actual.push(key) });

      if (keysType !== 'Array') {
        keys = Array.prototype.slice.call(arguments);
      }
    } else {
      actual = _.getOwnEnumerableProperties(obj);

      switch (keysType) {
        case 'Array':
          if (arguments.length > 1) {
            throw new AssertionError(mixedArgsMsg, undefined, ssfi);
          }
          break;
        case 'Object':
          if (arguments.length > 1) {
            throw new AssertionError(mixedArgsMsg, undefined, ssfi);
          }
          keys = Object.keys(keys);
          break;
        default:
          keys = Array.prototype.slice.call(arguments);
      }

      // Only stringify non-Symbols because Symbols would become "Symbol()"
      keys = keys.map(function (val) {
        return typeof val === 'symbol' ? val : String(val);
      });
    }

    if (!keys.length) {
      throw new AssertionError(flagMsg + 'keys required', undefined, ssfi);
    }

    var len = keys.length
      , any = flag(this, 'any')
      , all = flag(this, 'all')
      , expected = keys
      , isEql = isDeep ? flag(this, 'eql') : (val1, val2) => val1 === val2;

    if (!any && !all) {
      all = true;
    }

    // Has any
    if (any) {
      ok = expected.some(function(expectedKey) {
        return actual.some(function(actualKey) {
          return isEql(expectedKey, actualKey);
        });
      });
    }

    // Has all
    if (all) {
      ok = expected.every(function(expectedKey) {
        return actual.some(function(actualKey) {
          return isEql(expectedKey, actualKey);
        });
      });

      if (!flag(this, 'contains')) {
        ok = ok && keys.length == actual.length;
      }
    }

    // Key string
    if (len > 1) {
      keys = keys.map(function(key) {
        return _.inspect(key);
      });
      var last = keys.pop();
      if (all) {
        str = keys.join(', ') + ', and ' + last;
      }
      if (any) {
        str = keys.join(', ') + ', or ' + last;
      }
    } else {
      str = _.inspect(keys[0]);
    }

    // Form
    str = (len > 1 ? 'keys ' : 'key ') + str;

    // Have / include
    str = (flag(this, 'contains') ? 'contain ' : 'have ') + str;

    // Assertion
    this.assert(
        ok
      , 'expected #{this} to ' + deepStr + str
      , 'expected #{this} to not ' + deepStr + str
      , expected.slice(0).sort(_.compareByInspect)
      , actual.sort(_.compareByInspect)
      , true
    );
  }

  Assertion.addMethod('keys', assertKeys);
  Assertion.addMethod('key', assertKeys);

  /**
   * ### .throw([errorLike], [errMsgMatcher], [msg])
   *
   * When no arguments are provided, `.throw` invokes the target function and
   * asserts that an error is thrown.
   *
   *     var badFn = function () { throw new TypeError('Illegal salmon!'); };
   *
   *     expect(badFn).to.throw();
   *
   * When one argument is provided, and it's an error constructor, `.throw`
   * invokes the target function and asserts that an error is thrown that's an
   * instance of that error constructor.
   *
   *     var badFn = function () { throw new TypeError('Illegal salmon!'); };
   *
   *     expect(badFn).to.throw(TypeError);
   *
   * When one argument is provided, and it's an error instance, `.throw` invokes
   * the target function and asserts that an error is thrown that's strictly
   * (`===`) equal to that error instance.
   *
   *     var err = new TypeError('Illegal salmon!');
   *     var badFn = function () { throw err; };
   *
   *     expect(badFn).to.throw(err);
   *
   * When one argument is provided, and it's a string, `.throw` invokes the
   * target function and asserts that an error is thrown with a message that
   * contains that string.
   *
   *     var badFn = function () { throw new TypeError('Illegal salmon!'); };
   *
   *     expect(badFn).to.throw('salmon');
   *
   * When one argument is provided, and it's a regular expression, `.throw`
   * invokes the target function and asserts that an error is thrown with a
   * message that matches that regular expression.
   *
   *     var badFn = function () { throw new TypeError('Illegal salmon!'); };
   *
   *     expect(badFn).to.throw(/salmon/);
   *
   * When two arguments are provided, and the first is an error instance or
   * constructor, and the second is a string or regular expression, `.throw`
   * invokes the function and asserts that an error is thrown that fulfills both
   * conditions as described above.
   *
   *     var err = new TypeError('Illegal salmon!');
   *     var badFn = function () { throw err; };
   *
   *     expect(badFn).to.throw(TypeError, 'salmon');
   *     expect(badFn).to.throw(TypeError, /salmon/);
   *     expect(badFn).to.throw(err, 'salmon');
   *     expect(badFn).to.throw(err, /salmon/);
   *
   * Add `.not` earlier in the chain to negate `.throw`.
   *
   *     var goodFn = function () {};
   *
   *     expect(goodFn).to.not.throw();
   *
   * However, it's dangerous to negate `.throw` when providing any arguments.
   * The problem is that it creates uncertain expectations by asserting that the
   * target either doesn't throw an error, or that it throws an error but of a
   * different type than the given type, or that it throws an error of the given
   * type but with a message that doesn't include the given string. It's often
   * best to identify the exact output that's expected, and then write an
   * assertion that only accepts that exact output.
   *
   * When the target isn't expected to throw an error, it's often best to assert
   * exactly that.
   *
   *     var goodFn = function () {};
   *
   *     expect(goodFn).to.not.throw(); // Recommended
   *     expect(goodFn).to.not.throw(ReferenceError, 'x'); // Not recommended
   *
   * When the target is expected to throw an error, it's often best to assert
   * that the error is of its expected type, and has a message that includes an
   * expected string, rather than asserting that it doesn't have one of many
   * unexpected types, and doesn't have a message that includes some string.
   *
   *     var badFn = function () { throw new TypeError('Illegal salmon!'); };
   *
   *     expect(badFn).to.throw(TypeError, 'salmon'); // Recommended
   *     expect(badFn).to.not.throw(ReferenceError, 'x'); // Not recommended
   *
   * `.throw` changes the target of any assertions that follow in the chain to
   * be the error object that's thrown.
   *
   *     var err = new TypeError('Illegal salmon!');
   *     err.code = 42;
   *     var badFn = function () { throw err; };
   *
   *     expect(badFn).to.throw(TypeError).with.property('code', 42);
   *
   * `.throw` accepts an optional `msg` argument which is a custom error message
   * to show when the assertion fails. The message can also be given as the
   * second argument to `expect`. When not providing two arguments, always use
   * the second form.
   *
   *     var goodFn = function () {};
   *
   *     expect(goodFn).to.throw(TypeError, 'x', 'nooo why fail??');
   *     expect(goodFn, 'nooo why fail??').to.throw();
   *
   * Due to limitations in ES5, `.throw` may not always work as expected when
   * using a transpiler such as Babel or TypeScript. In particular, it may
   * produce unexpected results when subclassing the built-in `Error` object and
   * then passing the subclassed constructor to `.throw`. See your transpiler's
   * docs for details:
   *
   * - ([Babel](https://babeljs.io/docs/usage/caveats/#classes))
   * - ([TypeScript](https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#extending-built-ins-like-error-array-and-map-may-no-longer-work))
   *
   * Beware of some common mistakes when using the `throw` assertion. One common
   * mistake is to accidentally invoke the function yourself instead of letting
   * the `throw` assertion invoke the function for you. For example, when
   * testing if a function named `fn` throws, provide `fn` instead of `fn()` as
   * the target for the assertion.
   *
   *     expect(fn).to.throw();     // Good! Tests `fn` as desired
   *     expect(fn()).to.throw();   // Bad! Tests result of `fn()`, not `fn`
   *
   * If you need to assert that your function `fn` throws when passed certain
   * arguments, then wrap a call to `fn` inside of another function.
   *
   *     expect(function () { fn(42); }).to.throw();  // Function expression
   *     expect(() => fn(42)).to.throw();             // ES6 arrow function
   *
   * Another common mistake is to provide an object method (or any stand-alone
   * function that relies on `this`) as the target of the assertion. Doing so is
   * problematic because the `this` context will be lost when the function is
   * invoked by `.throw`; there's no way for it to know what `this` is supposed
   * to be. There are two ways around this problem. One solution is to wrap the
   * method or function call inside of another function. Another solution is to
   * use `bind`.
   *
   *     expect(function () { cat.meow(); }).to.throw();  // Function expression
   *     expect(() => cat.meow()).to.throw();             // ES6 arrow function
   *     expect(cat.meow.bind(cat)).to.throw();           // Bind
   *
   * Finally, it's worth mentioning that it's a best practice in JavaScript to
   * only throw `Error` and derivatives of `Error` such as `ReferenceError`,
   * `TypeError`, and user-defined objects that extend `Error`. No other type of
   * value will generate a stack trace when initialized. With that said, the
   * `throw` assertion does technically support any type of value being thrown,
   * not just `Error` and its derivatives.
   *
   * The aliases `.throws` and `.Throw` can be used interchangeably with
   * `.throw`.
   *
   * @name throw
   * @alias throws
   * @alias Throw
   * @param {Error|ErrorConstructor} errorLike
   * @param {String|RegExp} errMsgMatcher error message
   * @param {String} msg _optional_
   * @see https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Error#Error_types
   * @returns error for chaining (null if no error)
   * @namespace BDD
   * @api public
   */

  function assertThrows (errorLike, errMsgMatcher, msg) {
    if (msg) flag(this, 'message', msg);
    var obj = flag(this, 'object')
      , ssfi = flag(this, 'ssfi')
      , flagMsg = flag(this, 'message')
      , negate = flag(this, 'negate') || false;
    new Assertion(obj, flagMsg, ssfi, true).is.a('function');

    if (errorLike instanceof RegExp || typeof errorLike === 'string') {
      errMsgMatcher = errorLike;
      errorLike = null;
    }

    var caughtErr;
    try {
      obj();
    } catch (err) {
      caughtErr = err;
    }

    // If we have the negate flag enabled and at least one valid argument it means we do expect an error
    // but we want it to match a given set of criteria
    var everyArgIsUndefined = errorLike === undefined && errMsgMatcher === undefined;

    // If we've got the negate flag enabled and both args, we should only fail if both aren't compatible
    // See Issue #551 and PR #683@GitHub
    var everyArgIsDefined = Boolean(errorLike && errMsgMatcher);
    var errorLikeFail = false;
    var errMsgMatcherFail = false;

    // Checking if error was thrown
    if (everyArgIsUndefined || !everyArgIsUndefined && !negate) {
      // We need this to display results correctly according to their types
      var errorLikeString = 'an error';
      if (errorLike instanceof Error) {
        errorLikeString = '#{exp}';
      } else if (errorLike) {
        errorLikeString = _.checkError.getConstructorName(errorLike);
      }

      this.assert(
          caughtErr
        , 'expected #{this} to throw ' + errorLikeString
        , 'expected #{this} to not throw an error but #{act} was thrown'
        , errorLike && errorLike.toString()
        , (caughtErr instanceof Error ?
            caughtErr.toString() : (typeof caughtErr === 'string' ? caughtErr : caughtErr &&
                                    _.checkError.getConstructorName(caughtErr)))
      );
    }

    if (errorLike && caughtErr) {
      // We should compare instances only if `errorLike` is an instance of `Error`
      if (errorLike instanceof Error) {
        var isCompatibleInstance = _.checkError.compatibleInstance(caughtErr, errorLike);

        if (isCompatibleInstance === negate) {
          // These checks were created to ensure we won't fail too soon when we've got both args and a negate
          // See Issue #551 and PR #683@GitHub
          if (everyArgIsDefined && negate) {
            errorLikeFail = true;
          } else {
            this.assert(
                negate
              , 'expected #{this} to throw #{exp} but #{act} was thrown'
              , 'expected #{this} to not throw #{exp}' + (caughtErr && !negate ? ' but #{act} was thrown' : '')
              , errorLike.toString()
              , caughtErr.toString()
            );
          }
        }
      }

      var isCompatibleConstructor = _.checkError.compatibleConstructor(caughtErr, errorLike);
      if (isCompatibleConstructor === negate) {
        if (everyArgIsDefined && negate) {
            errorLikeFail = true;
        } else {
          this.assert(
              negate
            , 'expected #{this} to throw #{exp} but #{act} was thrown'
            , 'expected #{this} to not throw #{exp}' + (caughtErr ? ' but #{act} was thrown' : '')
            , (errorLike instanceof Error ? errorLike.toString() : errorLike && _.checkError.getConstructorName(errorLike))
            , (caughtErr instanceof Error ? caughtErr.toString() : caughtErr && _.checkError.getConstructorName(caughtErr))
          );
        }
      }
    }

    if (caughtErr && errMsgMatcher !== undefined && errMsgMatcher !== null) {
      // Here we check compatible messages
      var placeholder = 'including';
      if (errMsgMatcher instanceof RegExp) {
        placeholder = 'matching'
      }

      var isCompatibleMessage = _.checkError.compatibleMessage(caughtErr, errMsgMatcher);
      if (isCompatibleMessage === negate) {
        if (everyArgIsDefined && negate) {
            errMsgMatcherFail = true;
        } else {
          this.assert(
            negate
            , 'expected #{this} to throw error ' + placeholder + ' #{exp} but got #{act}'
            , 'expected #{this} to throw error not ' + placeholder + ' #{exp}'
            ,  errMsgMatcher
            ,  _.checkError.getMessage(caughtErr)
          );
        }
      }
    }

    // If both assertions failed and both should've matched we throw an error
    if (errorLikeFail && errMsgMatcherFail) {
      this.assert(
        negate
        , 'expected #{this} to throw #{exp} but #{act} was thrown'
        , 'expected #{this} to not throw #{exp}' + (caughtErr ? ' but #{act} was thrown' : '')
        , (errorLike instanceof Error ? errorLike.toString() : errorLike && _.checkError.getConstructorName(errorLike))
        , (caughtErr instanceof Error ? caughtErr.toString() : caughtErr && _.checkError.getConstructorName(caughtErr))
      );
    }

    flag(this, 'object', caughtErr);
  };

  Assertion.addMethod('throw', assertThrows);
  Assertion.addMethod('throws', assertThrows);
  Assertion.addMethod('Throw', assertThrows);

  /**
   * ### .respondTo(method[, msg])
   *
   * When the target is a non-function object, `.respondTo` asserts that the
   * target has a method with the given name `method`. The method can be own or
   * inherited, and it can be enumerable or non-enumerable.
   *
   *     function Cat () {}
   *     Cat.prototype.meow = function () {};
   *
   *     expect(new Cat()).to.respondTo('meow');
   *
   * When the target is a function, `.respondTo` asserts that the target's
   * `prototype` property has a method with the given name `method`. Again, the
   * method can be own or inherited, and it can be enumerable or non-enumerable.
   *
   *     function Cat () {}
   *     Cat.prototype.meow = function () {};
   *
   *     expect(Cat).to.respondTo('meow');
   *
   * Add `.itself` earlier in the chain to force `.respondTo` to treat the
   * target as a non-function object, even if it's a function. Thus, it asserts
   * that the target has a method with the given name `method`, rather than
   * asserting that the target's `prototype` property has a method with the
   * given name `method`.
   *
   *     function Cat () {}
   *     Cat.prototype.meow = function () {};
   *     Cat.hiss = function () {};
   *
   *     expect(Cat).itself.to.respondTo('hiss').but.not.respondTo('meow');
   *
   * When not adding `.itself`, it's important to check the target's type before
   * using `.respondTo`. See the `.a` doc for info on checking a target's type.
   *
   *     function Cat () {}
   *     Cat.prototype.meow = function () {};
   *
   *     expect(new Cat()).to.be.an('object').that.respondsTo('meow');
   *
   * Add `.not` earlier in the chain to negate `.respondTo`.
   *
   *     function Dog () {}
   *     Dog.prototype.bark = function () {};
   *
   *     expect(new Dog()).to.not.respondTo('meow');
   *
   * `.respondTo` accepts an optional `msg` argument which is a custom error
   * message to show when the assertion fails. The message can also be given as
   * the second argument to `expect`.
   *
   *     expect({}).to.respondTo('meow', 'nooo why fail??');
   *     expect({}, 'nooo why fail??').to.respondTo('meow');
   *
   * The alias `.respondsTo` can be used interchangeably with `.respondTo`.
   *
   * @name respondTo
   * @alias respondsTo
   * @param {String} method
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function respondTo (method, msg) {
    if (msg) flag(this, 'message', msg);
    var obj = flag(this, 'object')
      , itself = flag(this, 'itself')
      , context = ('function' === typeof obj && !itself)
        ? obj.prototype[method]
        : obj[method];

    this.assert(
        'function' === typeof context
      , 'expected #{this} to respond to ' + _.inspect(method)
      , 'expected #{this} to not respond to ' + _.inspect(method)
    );
  }

  Assertion.addMethod('respondTo', respondTo);
  Assertion.addMethod('respondsTo', respondTo);

  /**
   * ### .itself
   *
   * Forces all `.respondTo` assertions that follow in the chain to behave as if
   * the target is a non-function object, even if it's a function. Thus, it
   * causes `.respondTo` to assert that the target has a method with the given
   * name, rather than asserting that the target's `prototype` property has a
   * method with the given name.
   *
   *     function Cat () {}
   *     Cat.prototype.meow = function () {};
   *     Cat.hiss = function () {};
   *
   *     expect(Cat).itself.to.respondTo('hiss').but.not.respondTo('meow');
   *
   * @name itself
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('itself', function () {
    flag(this, 'itself', true);
  });

  /**
   * ### .satisfy(matcher[, msg])
   *
   * Invokes the given `matcher` function with the target being passed as the
   * first argument, and asserts that the value returned is truthy.
   *
   *     expect(1).to.satisfy(function(num) {
   *       return num > 0;
   *     });
   *
   * Add `.not` earlier in the chain to negate `.satisfy`.
   *
   *     expect(1).to.not.satisfy(function(num) {
   *       return num > 2;
   *     });
   *
   * `.satisfy` accepts an optional `msg` argument which is a custom error
   * message to show when the assertion fails. The message can also be given as
   * the second argument to `expect`.
   *
   *     expect(1).to.satisfy(function(num) {
   *       return num > 2;
   *     }, 'nooo why fail??');
   *
   *     expect(1, 'nooo why fail??').to.satisfy(function(num) {
   *       return num > 2;
   *     });
   *
   * The alias `.satisfies` can be used interchangeably with `.satisfy`.
   *
   * @name satisfy
   * @alias satisfies
   * @param {Function} matcher
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function satisfy (matcher, msg) {
    if (msg) flag(this, 'message', msg);
    var obj = flag(this, 'object');
    var result = matcher(obj);
    this.assert(
        result
      , 'expected #{this} to satisfy ' + _.objDisplay(matcher)
      , 'expected #{this} to not satisfy' + _.objDisplay(matcher)
      , flag(this, 'negate') ? false : true
      , result
    );
  }

  Assertion.addMethod('satisfy', satisfy);
  Assertion.addMethod('satisfies', satisfy);

  /**
   * ### .closeTo(expected, delta[, msg])
   *
   * Asserts that the target is a number that's within a given +/- `delta` range
   * of the given number `expected`. However, it's often best to assert that the
   * target is equal to its expected value.
   *
   *     // Recommended
   *     expect(1.5).to.equal(1.5);
   *
   *     // Not recommended
   *     expect(1.5).to.be.closeTo(1, 0.5);
   *     expect(1.5).to.be.closeTo(2, 0.5);
   *     expect(1.5).to.be.closeTo(1, 1);
   *
   * Add `.not` earlier in the chain to negate `.closeTo`.
   *
   *     expect(1.5).to.equal(1.5); // Recommended
   *     expect(1.5).to.not.be.closeTo(3, 1); // Not recommended
   *
   * `.closeTo` accepts an optional `msg` argument which is a custom error
   * message to show when the assertion fails. The message can also be given as
   * the second argument to `expect`.
   *
   *     expect(1.5).to.be.closeTo(3, 1, 'nooo why fail??');
   *     expect(1.5, 'nooo why fail??').to.be.closeTo(3, 1);
   *
   * The alias `.approximately` can be used interchangeably with `.closeTo`.
   *
   * @name closeTo
   * @alias approximately
   * @param {Number} expected
   * @param {Number} delta
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function closeTo(expected, delta, msg) {
    if (msg) flag(this, 'message', msg);
    var obj = flag(this, 'object')
      , flagMsg = flag(this, 'message')
      , ssfi = flag(this, 'ssfi');

    new Assertion(obj, flagMsg, ssfi, true).is.a('number');
    if (typeof expected !== 'number' || typeof delta !== 'number') {
      flagMsg = flagMsg ? flagMsg + ': ' : '';
      var deltaMessage = delta === undefined ? ", and a delta is required" : "";
      throw new AssertionError(
          flagMsg + 'the arguments to closeTo or approximately must be numbers' + deltaMessage,
          undefined,
          ssfi
      );
    }

    this.assert(
        Math.abs(obj - expected) <= delta
      , 'expected #{this} to be close to ' + expected + ' +/- ' + delta
      , 'expected #{this} not to be close to ' + expected + ' +/- ' + delta
    );
  }

  Assertion.addMethod('closeTo', closeTo);
  Assertion.addMethod('approximately', closeTo);

  // Note: Duplicates are ignored if testing for inclusion instead of sameness.
  function isSubsetOf(subset, superset, cmp, contains, ordered) {
    if (!contains) {
      if (subset.length !== superset.length) return false;
      superset = superset.slice();
    }

    return subset.every(function(elem, idx) {
      if (ordered) return cmp ? cmp(elem, superset[idx]) : elem === superset[idx];

      if (!cmp) {
        var matchIdx = superset.indexOf(elem);
        if (matchIdx === -1) return false;

        // Remove match from superset so not counted twice if duplicate in subset.
        if (!contains) superset.splice(matchIdx, 1);
        return true;
      }

      return superset.some(function(elem2, matchIdx) {
        if (!cmp(elem, elem2)) return false;

        // Remove match from superset so not counted twice if duplicate in subset.
        if (!contains) superset.splice(matchIdx, 1);
        return true;
      });
    });
  }

  /**
   * ### .members(set[, msg])
   *
   * Asserts that the target array has the same members as the given array
   * `set`.
   *
   *     expect([1, 2, 3]).to.have.members([2, 1, 3]);
   *     expect([1, 2, 2]).to.have.members([2, 1, 2]);
   *
   * By default, members are compared using strict (`===`) equality. Add `.deep`
   * earlier in the chain to use deep equality instead. See the `deep-eql`
   * project page for info on the deep equality algorithm:
   * https://github.com/chaijs/deep-eql.
   *
   *     // Target array deeply (but not strictly) has member `{a: 1}`
   *     expect([{a: 1}]).to.have.deep.members([{a: 1}]);
   *     expect([{a: 1}]).to.not.have.members([{a: 1}]);
   *
   * By default, order doesn't matter. Add `.ordered` earlier in the chain to
   * require that members appear in the same order.
   *
   *     expect([1, 2, 3]).to.have.ordered.members([1, 2, 3]);
   *     expect([1, 2, 3]).to.have.members([2, 1, 3])
   *       .but.not.ordered.members([2, 1, 3]);
   *
   * By default, both arrays must be the same size. Add `.include` earlier in
   * the chain to require that the target's members be a superset of the
   * expected members. Note that duplicates are ignored in the subset when
   * `.include` is added.
   *
   *     // Target array is a superset of [1, 2] but not identical
   *     expect([1, 2, 3]).to.include.members([1, 2]);
   *     expect([1, 2, 3]).to.not.have.members([1, 2]);
   *
   *     // Duplicates in the subset are ignored
   *     expect([1, 2, 3]).to.include.members([1, 2, 2, 2]);
   *
   * `.deep`, `.ordered`, and `.include` can all be combined. However, if
   * `.include` and `.ordered` are combined, the ordering begins at the start of
   * both arrays.
   *
   *     expect([{a: 1}, {b: 2}, {c: 3}])
   *       .to.include.deep.ordered.members([{a: 1}, {b: 2}])
   *       .but.not.include.deep.ordered.members([{b: 2}, {c: 3}]);
   *
   * Add `.not` earlier in the chain to negate `.members`. However, it's
   * dangerous to do so. The problem is that it creates uncertain expectations
   * by asserting that the target array doesn't have all of the same members as
   * the given array `set` but may or may not have some of them. It's often best
   * to identify the exact output that's expected, and then write an assertion
   * that only accepts that exact output.
   *
   *     expect([1, 2]).to.not.include(3).and.not.include(4); // Recommended
   *     expect([1, 2]).to.not.have.members([3, 4]); // Not recommended
   *
   * `.members` accepts an optional `msg` argument which is a custom error
   * message to show when the assertion fails. The message can also be given as
   * the second argument to `expect`.
   *
   *     expect([1, 2]).to.have.members([1, 2, 3], 'nooo why fail??');
   *     expect([1, 2], 'nooo why fail??').to.have.members([1, 2, 3]);
   *
   * @name members
   * @param {Array} set
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  Assertion.addMethod('members', function (subset, msg) {
    if (msg) flag(this, 'message', msg);
    var obj = flag(this, 'object')
      , flagMsg = flag(this, 'message')
      , ssfi = flag(this, 'ssfi');

    new Assertion(obj, flagMsg, ssfi, true).to.be.an('array');
    new Assertion(subset, flagMsg, ssfi, true).to.be.an('array');

    var contains = flag(this, 'contains');
    var ordered = flag(this, 'ordered');

    var subject, failMsg, failNegateMsg;

    if (contains) {
      subject = ordered ? 'an ordered superset' : 'a superset';
      failMsg = 'expected #{this} to be ' + subject + ' of #{exp}';
      failNegateMsg = 'expected #{this} to not be ' + subject + ' of #{exp}';
    } else {
      subject = ordered ? 'ordered members' : 'members';
      failMsg = 'expected #{this} to have the same ' + subject + ' as #{exp}';
      failNegateMsg = 'expected #{this} to not have the same ' + subject + ' as #{exp}';
    }

    var cmp = flag(this, 'deep') ? flag(this, 'eql') : undefined;

    this.assert(
        isSubsetOf(subset, obj, cmp, contains, ordered)
      , failMsg
      , failNegateMsg
      , subset
      , obj
      , true
    );
  });

  /**
   * ### .oneOf(list[, msg])
   *
   * Asserts that the target is a member of the given array `list`. However,
   * it's often best to assert that the target is equal to its expected value.
   *
   *     expect(1).to.equal(1); // Recommended
   *     expect(1).to.be.oneOf([1, 2, 3]); // Not recommended
   *
   * Comparisons are performed using strict (`===`) equality.
   *
   * Add `.not` earlier in the chain to negate `.oneOf`.
   *
   *     expect(1).to.equal(1); // Recommended
   *     expect(1).to.not.be.oneOf([2, 3, 4]); // Not recommended
   *
   * It can also be chained with `.contain` or `.include`, which will work with
   * both arrays and strings:
   *
   *     expect('Today is sunny').to.contain.oneOf(['sunny', 'cloudy'])
   *     expect('Today is rainy').to.not.contain.oneOf(['sunny', 'cloudy'])
   *     expect([1,2,3]).to.contain.oneOf([3,4,5])
   *     expect([1,2,3]).to.not.contain.oneOf([4,5,6])
   *
   * `.oneOf` accepts an optional `msg` argument which is a custom error message
   * to show when the assertion fails. The message can also be given as the
   * second argument to `expect`.
   *
   *     expect(1).to.be.oneOf([2, 3, 4], 'nooo why fail??');
   *     expect(1, 'nooo why fail??').to.be.oneOf([2, 3, 4]);
   *
   * @name oneOf
   * @param {Array<*>} list
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function oneOf (list, msg) {
    if (msg) flag(this, 'message', msg);
    var expected = flag(this, 'object')
      , flagMsg = flag(this, 'message')
      , ssfi = flag(this, 'ssfi')
      , contains = flag(this, 'contains')
      , isDeep = flag(this, 'deep')
      , eql = flag(this, 'eql');
    new Assertion(list, flagMsg, ssfi, true).to.be.an('array');

    if (contains) {
      this.assert(
        list.some(function(possibility) { return expected.indexOf(possibility) > -1 })
        , 'expected #{this} to contain one of #{exp}'
        , 'expected #{this} to not contain one of #{exp}'
        , list
        , expected
      );
    } else {
      if (isDeep) {
        this.assert(
          list.some(function(possibility) { return eql(expected, possibility) })
          , 'expected #{this} to deeply equal one of #{exp}'
          , 'expected #{this} to deeply equal one of #{exp}'
          , list
          , expected
        );
      } else {
        this.assert(
          list.indexOf(expected) > -1
          , 'expected #{this} to be one of #{exp}'
          , 'expected #{this} to not be one of #{exp}'
          , list
          , expected
        );
      }
    }
  }

  Assertion.addMethod('oneOf', oneOf);

  /**
   * ### .change(subject[, prop[, msg]])
   *
   * When one argument is provided, `.change` asserts that the given function
   * `subject` returns a different value when it's invoked before the target
   * function compared to when it's invoked afterward. However, it's often best
   * to assert that `subject` is equal to its expected value.
   *
   *     var dots = ''
   *       , addDot = function () { dots += '.'; }
   *       , getDots = function () { return dots; };
   *
   *     // Recommended
   *     expect(getDots()).to.equal('');
   *     addDot();
   *     expect(getDots()).to.equal('.');
   *
   *     // Not recommended
   *     expect(addDot).to.change(getDots);
   *
   * When two arguments are provided, `.change` asserts that the value of the
   * given object `subject`'s `prop` property is different before invoking the
   * target function compared to afterward.
   *
   *     var myObj = {dots: ''}
   *       , addDot = function () { myObj.dots += '.'; };
   *
   *     // Recommended
   *     expect(myObj).to.have.property('dots', '');
   *     addDot();
   *     expect(myObj).to.have.property('dots', '.');
   *
   *     // Not recommended
   *     expect(addDot).to.change(myObj, 'dots');
   *
   * Strict (`===`) equality is used to compare before and after values.
   *
   * Add `.not` earlier in the chain to negate `.change`.
   *
   *     var dots = ''
   *       , noop = function () {}
   *       , getDots = function () { return dots; };
   *
   *     expect(noop).to.not.change(getDots);
   *
   *     var myObj = {dots: ''}
   *       , noop = function () {};
   *
   *     expect(noop).to.not.change(myObj, 'dots');
   *
   * `.change` accepts an optional `msg` argument which is a custom error
   * message to show when the assertion fails. The message can also be given as
   * the second argument to `expect`. When not providing two arguments, always
   * use the second form.
   *
   *     var myObj = {dots: ''}
   *       , addDot = function () { myObj.dots += '.'; };
   *
   *     expect(addDot).to.not.change(myObj, 'dots', 'nooo why fail??');
   *
   *     var dots = ''
   *       , addDot = function () { dots += '.'; }
   *       , getDots = function () { return dots; };
   *
   *     expect(addDot, 'nooo why fail??').to.not.change(getDots);
   *
   * `.change` also causes all `.by` assertions that follow in the chain to
   * assert how much a numeric subject was increased or decreased by. However,
   * it's dangerous to use `.change.by`. The problem is that it creates
   * uncertain expectations by asserting that the subject either increases by
   * the given delta, or that it decreases by the given delta. It's often best
   * to identify the exact output that's expected, and then write an assertion
   * that only accepts that exact output.
   *
   *     var myObj = {val: 1}
   *       , addTwo = function () { myObj.val += 2; }
   *       , subtractTwo = function () { myObj.val -= 2; };
   *
   *     expect(addTwo).to.increase(myObj, 'val').by(2); // Recommended
   *     expect(addTwo).to.change(myObj, 'val').by(2); // Not recommended
   *
   *     expect(subtractTwo).to.decrease(myObj, 'val').by(2); // Recommended
   *     expect(subtractTwo).to.change(myObj, 'val').by(2); // Not recommended
   *
   * The alias `.changes` can be used interchangeably with `.change`.
   *
   * @name change
   * @alias changes
   * @param {String} subject
   * @param {String} prop name _optional_
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function assertChanges (subject, prop, msg) {
    if (msg) flag(this, 'message', msg);
    var fn = flag(this, 'object')
      , flagMsg = flag(this, 'message')
      , ssfi = flag(this, 'ssfi');
    new Assertion(fn, flagMsg, ssfi, true).is.a('function');

    var initial;
    if (!prop) {
      new Assertion(subject, flagMsg, ssfi, true).is.a('function');
      initial = subject();
    } else {
      new Assertion(subject, flagMsg, ssfi, true).to.have.property(prop);
      initial = subject[prop];
    }

    fn();

    var final = prop === undefined || prop === null ? subject() : subject[prop];
    var msgObj = prop === undefined || prop === null ? initial : '.' + prop;

    // This gets flagged because of the .by(delta) assertion
    flag(this, 'deltaMsgObj', msgObj);
    flag(this, 'initialDeltaValue', initial);
    flag(this, 'finalDeltaValue', final);
    flag(this, 'deltaBehavior', 'change');
    flag(this, 'realDelta', final !== initial);

    this.assert(
      initial !== final
      , 'expected ' + msgObj + ' to change'
      , 'expected ' + msgObj + ' to not change'
    );
  }

  Assertion.addMethod('change', assertChanges);
  Assertion.addMethod('changes', assertChanges);

  /**
   * ### .increase(subject[, prop[, msg]])
   *
   * When one argument is provided, `.increase` asserts that the given function
   * `subject` returns a greater number when it's invoked after invoking the
   * target function compared to when it's invoked beforehand. `.increase` also
   * causes all `.by` assertions that follow in the chain to assert how much
   * greater of a number is returned. It's often best to assert that the return
   * value increased by the expected amount, rather than asserting it increased
   * by any amount.
   *
   *     var val = 1
   *       , addTwo = function () { val += 2; }
   *       , getVal = function () { return val; };
   *
   *     expect(addTwo).to.increase(getVal).by(2); // Recommended
   *     expect(addTwo).to.increase(getVal); // Not recommended
   *
   * When two arguments are provided, `.increase` asserts that the value of the
   * given object `subject`'s `prop` property is greater after invoking the
   * target function compared to beforehand.
   *
   *     var myObj = {val: 1}
   *       , addTwo = function () { myObj.val += 2; };
   *
   *     expect(addTwo).to.increase(myObj, 'val').by(2); // Recommended
   *     expect(addTwo).to.increase(myObj, 'val'); // Not recommended
   *
   * Add `.not` earlier in the chain to negate `.increase`. However, it's
   * dangerous to do so. The problem is that it creates uncertain expectations
   * by asserting that the subject either decreases, or that it stays the same.
   * It's often best to identify the exact output that's expected, and then
   * write an assertion that only accepts that exact output.
   *
   * When the subject is expected to decrease, it's often best to assert that it
   * decreased by the expected amount.
   *
   *     var myObj = {val: 1}
   *       , subtractTwo = function () { myObj.val -= 2; };
   *
   *     expect(subtractTwo).to.decrease(myObj, 'val').by(2); // Recommended
   *     expect(subtractTwo).to.not.increase(myObj, 'val'); // Not recommended
   *
   * When the subject is expected to stay the same, it's often best to assert
   * exactly that.
   *
   *     var myObj = {val: 1}
   *       , noop = function () {};
   *
   *     expect(noop).to.not.change(myObj, 'val'); // Recommended
   *     expect(noop).to.not.increase(myObj, 'val'); // Not recommended
   *
   * `.increase` accepts an optional `msg` argument which is a custom error
   * message to show when the assertion fails. The message can also be given as
   * the second argument to `expect`. When not providing two arguments, always
   * use the second form.
   *
   *     var myObj = {val: 1}
   *       , noop = function () {};
   *
   *     expect(noop).to.increase(myObj, 'val', 'nooo why fail??');
   *
   *     var val = 1
   *       , noop = function () {}
   *       , getVal = function () { return val; };
   *
   *     expect(noop, 'nooo why fail??').to.increase(getVal);
   *
   * The alias `.increases` can be used interchangeably with `.increase`.
   *
   * @name increase
   * @alias increases
   * @param {String|Function} subject
   * @param {String} prop name _optional_
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function assertIncreases (subject, prop, msg) {
    if (msg) flag(this, 'message', msg);
    var fn = flag(this, 'object')
      , flagMsg = flag(this, 'message')
      , ssfi = flag(this, 'ssfi');
    new Assertion(fn, flagMsg, ssfi, true).is.a('function');

    var initial;
    if (!prop) {
      new Assertion(subject, flagMsg, ssfi, true).is.a('function');
      initial = subject();
    } else {
      new Assertion(subject, flagMsg, ssfi, true).to.have.property(prop);
      initial = subject[prop];
    }

    // Make sure that the target is a number
    new Assertion(initial, flagMsg, ssfi, true).is.a('number');

    fn();

    var final = prop === undefined || prop === null ? subject() : subject[prop];
    var msgObj = prop === undefined || prop === null ? initial : '.' + prop;

    flag(this, 'deltaMsgObj', msgObj);
    flag(this, 'initialDeltaValue', initial);
    flag(this, 'finalDeltaValue', final);
    flag(this, 'deltaBehavior', 'increase');
    flag(this, 'realDelta', final - initial);

    this.assert(
      final - initial > 0
      , 'expected ' + msgObj + ' to increase'
      , 'expected ' + msgObj + ' to not increase'
    );
  }

  Assertion.addMethod('increase', assertIncreases);
  Assertion.addMethod('increases', assertIncreases);

  /**
   * ### .decrease(subject[, prop[, msg]])
   *
   * When one argument is provided, `.decrease` asserts that the given function
   * `subject` returns a lesser number when it's invoked after invoking the
   * target function compared to when it's invoked beforehand. `.decrease` also
   * causes all `.by` assertions that follow in the chain to assert how much
   * lesser of a number is returned. It's often best to assert that the return
   * value decreased by the expected amount, rather than asserting it decreased
   * by any amount.
   *
   *     var val = 1
   *       , subtractTwo = function () { val -= 2; }
   *       , getVal = function () { return val; };
   *
   *     expect(subtractTwo).to.decrease(getVal).by(2); // Recommended
   *     expect(subtractTwo).to.decrease(getVal); // Not recommended
   *
   * When two arguments are provided, `.decrease` asserts that the value of the
   * given object `subject`'s `prop` property is lesser after invoking the
   * target function compared to beforehand.
   *
   *     var myObj = {val: 1}
   *       , subtractTwo = function () { myObj.val -= 2; };
   *
   *     expect(subtractTwo).to.decrease(myObj, 'val').by(2); // Recommended
   *     expect(subtractTwo).to.decrease(myObj, 'val'); // Not recommended
   *
   * Add `.not` earlier in the chain to negate `.decrease`. However, it's
   * dangerous to do so. The problem is that it creates uncertain expectations
   * by asserting that the subject either increases, or that it stays the same.
   * It's often best to identify the exact output that's expected, and then
   * write an assertion that only accepts that exact output.
   *
   * When the subject is expected to increase, it's often best to assert that it
   * increased by the expected amount.
   *
   *     var myObj = {val: 1}
   *       , addTwo = function () { myObj.val += 2; };
   *
   *     expect(addTwo).to.increase(myObj, 'val').by(2); // Recommended
   *     expect(addTwo).to.not.decrease(myObj, 'val'); // Not recommended
   *
   * When the subject is expected to stay the same, it's often best to assert
   * exactly that.
   *
   *     var myObj = {val: 1}
   *       , noop = function () {};
   *
   *     expect(noop).to.not.change(myObj, 'val'); // Recommended
   *     expect(noop).to.not.decrease(myObj, 'val'); // Not recommended
   *
   * `.decrease` accepts an optional `msg` argument which is a custom error
   * message to show when the assertion fails. The message can also be given as
   * the second argument to `expect`. When not providing two arguments, always
   * use the second form.
   *
   *     var myObj = {val: 1}
   *       , noop = function () {};
   *
   *     expect(noop).to.decrease(myObj, 'val', 'nooo why fail??');
   *
   *     var val = 1
   *       , noop = function () {}
   *       , getVal = function () { return val; };
   *
   *     expect(noop, 'nooo why fail??').to.decrease(getVal);
   *
   * The alias `.decreases` can be used interchangeably with `.decrease`.
   *
   * @name decrease
   * @alias decreases
   * @param {String|Function} subject
   * @param {String} prop name _optional_
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function assertDecreases (subject, prop, msg) {
    if (msg) flag(this, 'message', msg);
    var fn = flag(this, 'object')
      , flagMsg = flag(this, 'message')
      , ssfi = flag(this, 'ssfi');
    new Assertion(fn, flagMsg, ssfi, true).is.a('function');

    var initial;
    if (!prop) {
      new Assertion(subject, flagMsg, ssfi, true).is.a('function');
      initial = subject();
    } else {
      new Assertion(subject, flagMsg, ssfi, true).to.have.property(prop);
      initial = subject[prop];
    }

    // Make sure that the target is a number
    new Assertion(initial, flagMsg, ssfi, true).is.a('number');

    fn();

    var final = prop === undefined || prop === null ? subject() : subject[prop];
    var msgObj = prop === undefined || prop === null ? initial : '.' + prop;

    flag(this, 'deltaMsgObj', msgObj);
    flag(this, 'initialDeltaValue', initial);
    flag(this, 'finalDeltaValue', final);
    flag(this, 'deltaBehavior', 'decrease');
    flag(this, 'realDelta', initial - final);

    this.assert(
      final - initial < 0
      , 'expected ' + msgObj + ' to decrease'
      , 'expected ' + msgObj + ' to not decrease'
    );
  }

  Assertion.addMethod('decrease', assertDecreases);
  Assertion.addMethod('decreases', assertDecreases);

  /**
   * ### .by(delta[, msg])
   *
   * When following an `.increase` assertion in the chain, `.by` asserts that
   * the subject of the `.increase` assertion increased by the given `delta`.
   *
   *     var myObj = {val: 1}
   *       , addTwo = function () { myObj.val += 2; };
   *
   *     expect(addTwo).to.increase(myObj, 'val').by(2);
   *
   * When following a `.decrease` assertion in the chain, `.by` asserts that the
   * subject of the `.decrease` assertion decreased by the given `delta`.
   *
   *     var myObj = {val: 1}
   *       , subtractTwo = function () { myObj.val -= 2; };
   *
   *     expect(subtractTwo).to.decrease(myObj, 'val').by(2);
   *
   * When following a `.change` assertion in the chain, `.by` asserts that the
   * subject of the `.change` assertion either increased or decreased by the
   * given `delta`. However, it's dangerous to use `.change.by`. The problem is
   * that it creates uncertain expectations. It's often best to identify the
   * exact output that's expected, and then write an assertion that only accepts
   * that exact output.
   *
   *     var myObj = {val: 1}
   *       , addTwo = function () { myObj.val += 2; }
   *       , subtractTwo = function () { myObj.val -= 2; };
   *
   *     expect(addTwo).to.increase(myObj, 'val').by(2); // Recommended
   *     expect(addTwo).to.change(myObj, 'val').by(2); // Not recommended
   *
   *     expect(subtractTwo).to.decrease(myObj, 'val').by(2); // Recommended
   *     expect(subtractTwo).to.change(myObj, 'val').by(2); // Not recommended
   *
   * Add `.not` earlier in the chain to negate `.by`. However, it's often best
   * to assert that the subject changed by its expected delta, rather than
   * asserting that it didn't change by one of countless unexpected deltas.
   *
   *     var myObj = {val: 1}
   *       , addTwo = function () { myObj.val += 2; };
   *
   *     // Recommended
   *     expect(addTwo).to.increase(myObj, 'val').by(2);
   *
   *     // Not recommended
   *     expect(addTwo).to.increase(myObj, 'val').but.not.by(3);
   *
   * `.by` accepts an optional `msg` argument which is a custom error message to
   * show when the assertion fails. The message can also be given as the second
   * argument to `expect`.
   *
   *     var myObj = {val: 1}
   *       , addTwo = function () { myObj.val += 2; };
   *
   *     expect(addTwo).to.increase(myObj, 'val').by(3, 'nooo why fail??');
   *     expect(addTwo, 'nooo why fail??').to.increase(myObj, 'val').by(3);
   *
   * @name by
   * @param {Number} delta
   * @param {String} msg _optional_
   * @namespace BDD
   * @api public
   */

  function assertDelta(delta, msg) {
    if (msg) flag(this, 'message', msg);

    var msgObj = flag(this, 'deltaMsgObj');
    var initial = flag(this, 'initialDeltaValue');
    var final = flag(this, 'finalDeltaValue');
    var behavior = flag(this, 'deltaBehavior');
    var realDelta = flag(this, 'realDelta');

    var expression;
    if (behavior === 'change') {
      expression = Math.abs(final - initial) === Math.abs(delta);
    } else {
      expression = realDelta === Math.abs(delta);
    }

    this.assert(
      expression
      , 'expected ' + msgObj + ' to ' + behavior + ' by ' + delta
      , 'expected ' + msgObj + ' to not ' + behavior + ' by ' + delta
    );
  }

  Assertion.addMethod('by', assertDelta);

  /**
   * ### .extensible
   *
   * Asserts that the target is extensible, which means that new properties can
   * be added to it. Primitives are never extensible.
   *
   *     expect({a: 1}).to.be.extensible;
   *
   * Add `.not` earlier in the chain to negate `.extensible`.
   *
   *     var nonExtensibleObject = Object.preventExtensions({})
   *       , sealedObject = Object.seal({})
   *       , frozenObject = Object.freeze({});
   *
   *     expect(nonExtensibleObject).to.not.be.extensible;
   *     expect(sealedObject).to.not.be.extensible;
   *     expect(frozenObject).to.not.be.extensible;
   *     expect(1).to.not.be.extensible;
   *
   * A custom error message can be given as the second argument to `expect`.
   *
   *     expect(1, 'nooo why fail??').to.be.extensible;
   *
   * @name extensible
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('extensible', function() {
    var obj = flag(this, 'object');

    // In ES5, if the argument to this method is a primitive, then it will cause a TypeError.
    // In ES6, a non-object argument will be treated as if it was a non-extensible ordinary object, simply return false.
    // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isExtensible
    // The following provides ES6 behavior for ES5 environments.

    var isExtensible = obj === Object(obj) && Object.isExtensible(obj);

    this.assert(
      isExtensible
      , 'expected #{this} to be extensible'
      , 'expected #{this} to not be extensible'
    );
  });

  /**
   * ### .sealed
   *
   * Asserts that the target is sealed, which means that new properties can't be
   * added to it, and its existing properties can't be reconfigured or deleted.
   * However, it's possible that its existing properties can still be reassigned
   * to different values. Primitives are always sealed.
   *
   *     var sealedObject = Object.seal({});
   *     var frozenObject = Object.freeze({});
   *
   *     expect(sealedObject).to.be.sealed;
   *     expect(frozenObject).to.be.sealed;
   *     expect(1).to.be.sealed;
   *
   * Add `.not` earlier in the chain to negate `.sealed`.
   *
   *     expect({a: 1}).to.not.be.sealed;
   *
   * A custom error message can be given as the second argument to `expect`.
   *
   *     expect({a: 1}, 'nooo why fail??').to.be.sealed;
   *
   * @name sealed
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('sealed', function() {
    var obj = flag(this, 'object');

    // In ES5, if the argument to this method is a primitive, then it will cause a TypeError.
    // In ES6, a non-object argument will be treated as if it was a sealed ordinary object, simply return true.
    // See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isSealed
    // The following provides ES6 behavior for ES5 environments.

    var isSealed = obj === Object(obj) ? Object.isSealed(obj) : true;

    this.assert(
      isSealed
      , 'expected #{this} to be sealed'
      , 'expected #{this} to not be sealed'
    );
  });

  /**
   * ### .frozen
   *
   * Asserts that the target is frozen, which means that new properties can't be
   * added to it, and its existing properties can't be reassigned to different
   * values, reconfigured, or deleted. Primitives are always frozen.
   *
   *     var frozenObject = Object.freeze({});
   *
   *     expect(frozenObject).to.be.frozen;
   *     expect(1).to.be.frozen;
   *
   * Add `.not` earlier in the chain to negate `.frozen`.
   *
   *     expect({a: 1}).to.not.be.frozen;
   *
   * A custom error message can be given as the second argument to `expect`.
   *
   *     expect({a: 1}, 'nooo why fail??').to.be.frozen;
   *
   * @name frozen
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('frozen', function() {
    var obj = flag(this, 'object');

    // In ES5, if the argument to this method is a primitive, then it will cause a TypeError.
    // In ES6, a non-object argument will be treated as if it was a frozen ordinary object, simply return true.
    // See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isFrozen
    // The following provides ES6 behavior for ES5 environments.

    var isFrozen = obj === Object(obj) ? Object.isFrozen(obj) : true;

    this.assert(
      isFrozen
      , 'expected #{this} to be frozen'
      , 'expected #{this} to not be frozen'
    );
  });

  /**
   * ### .finite
   *
   * Asserts that the target is a number, and isn't `NaN` or positive/negative
   * `Infinity`.
   *
   *     expect(1).to.be.finite;
   *
   * Add `.not` earlier in the chain to negate `.finite`. However, it's
   * dangerous to do so. The problem is that it creates uncertain expectations
   * by asserting that the subject either isn't a number, or that it's `NaN`, or
   * that it's positive `Infinity`, or that it's negative `Infinity`. It's often
   * best to identify the exact output that's expected, and then write an
   * assertion that only accepts that exact output.
   *
   * When the target isn't expected to be a number, it's often best to assert
   * that it's the expected type, rather than asserting that it isn't one of
   * many unexpected types.
   *
   *     expect('foo').to.be.a('string'); // Recommended
   *     expect('foo').to.not.be.finite; // Not recommended
   *
   * When the target is expected to be `NaN`, it's often best to assert exactly
   * that.
   *
   *     expect(NaN).to.be.NaN; // Recommended
   *     expect(NaN).to.not.be.finite; // Not recommended
   *
   * When the target is expected to be positive infinity, it's often best to
   * assert exactly that.
   *
   *     expect(Infinity).to.equal(Infinity); // Recommended
   *     expect(Infinity).to.not.be.finite; // Not recommended
   *
   * When the target is expected to be negative infinity, it's often best to
   * assert exactly that.
   *
   *     expect(-Infinity).to.equal(-Infinity); // Recommended
   *     expect(-Infinity).to.not.be.finite; // Not recommended
   *
   * A custom error message can be given as the second argument to `expect`.
   *
   *     expect('foo', 'nooo why fail??').to.be.finite;
   *
   * @name finite
   * @namespace BDD
   * @api public
   */

  Assertion.addProperty('finite', function(msg) {
    var obj = flag(this, 'object');

    this.assert(
        typeof obj === 'number' && isFinite(obj)
      , 'expected #{this} to be a finite number'
      , 'expected #{this} to not be a finite number'
    );
  });
};