Add proxyExcludedKeys config setting. Closes chaijs#765

lib/chai/config.js

@@ -70,5 +70,25 @@ module.exports = {
    * @api public
    */
 
-  useProxy: true
+  useProxy: true,
+
+  /**
+   * ### config.proxyExcludedKeys
+   *
+   * User configurable property, defines which non-existing properties should be ignored
+   * instead of throwing an error if they do not exist on the assertion.
+   * This is only applied if the environment Chai is running into supports proxies and
+   * if the `useProxy` configuration setting is enabled.
+   *
+   *     // By default these keys will not throw an error if they do not exist on the assertion object
+   *     chai.config.proxyExcludedKeys = ['then', 'inspect'];
+   *
+   * This feature is automatically disabled regardless of this config value
+   * in environments that don't support proxies.
+   *
+   * @param {Array}
+   * @api public
+   */
+
+  proxyExcludedKeys: ['then', 'inspect']
 };

lib/chai/utils/proxify.js

@@ -26,8 +26,10 @@ module.exports = function proxify (obj) {
     get: function getProperty (target, property) {
       // Don't throw error on Symbol properties such as Symbol.toStringTag, nor
       // on .then because it's necessary for promise type-checking.
+      // The values for which an error should be thrown can be configured using
+      // the `config.proxyExcludedKeys` setting.
       if (typeof property === 'string' &&
-          property !== 'then' &&
+          config.proxyExcludedKeys.indexOf(property) === -1 &&
           !Reflect.has(target, property))
         throw Error('Invalid Chai property: ' + property);
 

test/configuration.js

@@ -196,4 +196,37 @@ describe('configuration', function () {
       });
     });
   });
+
+  describe('proxyExcludedKeys', function() {
+    var readNoExistentProperty = function(prop) {
+      return function() {
+        var assertion = expect(false);
+        expect(assertion).to.not.have.key(prop);
+        assertion[prop];
+      }
+    };
+
+    it('should have default value equal to `[\'then\', \'inspect\']`', function() {
+      expect(chai.config.proxyExcludedKeys).to.be.deep.equal(['then', 'inspect']);
+    });
+
+    it('should not throw when accessing non-existing `then` and `inspect` on an environment with proxy support', function() {
+      // Since these will not throw if the environment does not support proxies we don't need any `if` clause here
+      expect(readNoExistentProperty('then')).to.not.throw();
+      expect(readNoExistentProperty('inspect')).to.not.throw();
+    });
+
+    it('should throw for properties which are not on the `proxyExcludedKeys` Array on an environment with proxy support', function() {
+      chai.config.proxyExcludedKeys = [];
+
+      if (typeof Proxy !== 'undefined' && typeof Reflect !== 'undefined') {
+        expect(readNoExistentProperty('then')).to.throw('Invalid Chai property: then');
+        expect(readNoExistentProperty('inspect')).to.throw('Invalid Chai property: inspect');
+      } else {
+        expect(readNoExistentProperty('then')).to.not.throw();
+        expect(readNoExistentProperty('inspect')).to.not.throw();
+      }
+    });
+  });
+
 });