feat: chai.spy.callsBackWith implements #65

README.md

@@ -131,6 +131,15 @@ Better to use arrow function:
 const returnTrue = chai.spy(returns => true);
 ```
 
+### spy.callsBackWith
+`chai.spy.callsBackWith` is a helper which creates a function that calls the provided callback function with the provided values:
+
+```js
+// or you can create a spy which calls back with some values
+const spy = chai.spy.callsBackWith(new Error('foo'));
+const spy = chai.spy.callsBackWith(null, {id: 12});
+````
+
 ### Sandboxes
 
 Sandbox is a set of spies. Sandbox allows to track methods on objects and restore original methods with on `restore` call.

lib/spy.js

@@ -340,6 +340,30 @@ module.exports = function (chai, _) {
     });
   };
 
+  /**
+   * # chai.spy.callsBackWith (function)
+   *
+   * Creates a spy which automatically calls any passed callback function with the provided values.
+   *
+   *      var method = chai.spy.callsBackWith(new Error('foo'));
+   *      var method = chai.spy.callsBackWith(null, {id:12});
+   *
+   * @param {*} value static value which is returned by spy
+   * @returns new spy function which calls the 
+   * @api public
+   */
+
+  chai.spy.callsBackWith = function () {
+    var args = arguments;
+    return chai.spy(function(){
+      callbackFn = arguments[arguments.length-1];
+      chai.assert(
+          typeof callbackFn === 'function'
+        , 'expected ' + callbackFn + ' to be a callback function');
+      callbackFn.apply(null, args);
+    });
+  }
+
   /**
    * # spy
    *

test/spies.js

@@ -227,6 +227,35 @@ describe('Chai Spies', function () {
     spy().should.equal(value);
   });
 
+  it('should create spy which calls back with static values', function() {
+    var value1 = 7;
+    var value2 = true;
+    var value3 = {id:4};
+    var spy = chai.spy.callsBackWith(value1, value2, value3);
+
+    var callback = function(arg1, arg2, arg3) {
+      arg1.should.equal(value1);
+      arg2.should.equal(value2);
+      arg3.should.equal(value3);
+    };
+
+    spy.should.be.a.spy;
+    spy(123, false, callback);
+  });
+
+  it('should know if final argument is a function', function() {
+    var value = 'foo';
+    var spy = chai.spy.callsBackWith(value);
+
+    spy.should.be.a.spy;
+    (function(){
+      spy(3);
+    }).should.throw(chai.AssertionError);
+    (function(){
+      spy();
+    }).should.throw(chai.AssertionError);
+  });
+
   describe('.with', function () {
     it('should not interfere chai with' ,function () {
       (1).should.be.with.a('number');