Return new assertion instead of this for overwriteMethod

lib/chai/utils/overwriteMethod.js

@@ -4,7 +4,9 @@
  * MIT Licensed
  */
 
+var chai = require('../../chai');
 var flag = require('./flag');
+var transferFlags = require('./transferFlags');
 
 /**
  * ### overwriteMethod (ctx, name, fn)
@@ -59,6 +61,12 @@ module.exports = function (ctx, name, method) {
     var result = method(_super).apply(this, arguments);
     flag(this, 'keep_ssfi', false);
 
-    return result === undefined ? this : result;
+    if (result !== undefined) {
+      return result;
+    }
+
+    var newAssertion = new chai.Assertion();
+    transferFlags(this, newAssertion);
+    return newAssertion;
   }
 };

test/utilities.js

@@ -341,16 +341,23 @@ describe('utilities', function () {
   });
 
   describe('overwriteMethod', function () {
+    var assertionConstructor;
+
     before(function() {
       chai.config.includeStack = false;
 
-      chai.use(function(_chai, _) {
+      chai.use(function(_chai, utils) {
+        assertionConstructor = _chai.Assertion;
+
         _chai.Assertion.addMethod('four', function() {
           this.assert(this._obj === 4, 'expected #{this} to be 4', 'expected #{this} to not be 4', 4);
         });
 
         _chai.Assertion.overwriteMethod('four', function(_super) {
           return function() {
+            utils.flag(this, 'mySpecificFlag', 'value1');
+            utils.flag(this, 'ultraSpecificFlag', 'value2');
+
             if (typeof this._obj === 'string') {
               this.assert(this._obj === 'four', 'expected #{this} to be \'four\'', 'expected #{this} to not be \'four\'', 'four');
             } else {
@@ -358,6 +365,15 @@ describe('utilities', function () {
             }
           }
         });
+
+        _chai.Assertion.addMethod('checkFlags', function() {
+          this.assert(
+              utils.flag(this, 'mySpecificFlag') === 'value1' &&
+              utils.flag(this, 'ultraSpecificFlag') === 'value2'
+            , 'expected assertion to have specific flags'
+            , "this doesn't matter"
+          );
+        });
       });
     });
 
@@ -392,6 +408,26 @@ describe('utilities', function () {
         }
       }
     });
+
+    it('should return a new assertion with flags copied over', function () {
+      var assertion1 = expect('four');
+      var assertion2 = assertion1.four();
+
+      // Checking if a new assertion was returned
+      expect(assertion1).to.not.be.equal(assertion2);
+
+      // Check if flags were copied
+      assertion2.checkFlags();
+
+      // Checking if it's really an instance of an Assertion
+      expect(assertion2).to.be.instanceOf(assertionConstructor);
+
+      // Test chaining `.length` after a method to guarantee it is not a function's `length`
+      expect('four').to.be.a.four().length.above(2);
+
+      // Ensure that foo returns an Assertion (not a function)
+      expect(expect('four').four()).to.be.an.instanceOf(assertionConstructor);
+    });
   });
 
   it('addProperty', function () {