.overwriteMethod(ctx, name, fn)

Overwrites an already existing method and provides access to previous function. Must return function to be used for name.

utils.overwriteMethod(chai.Assertion.prototype, 'equal', function (_super) {
  return function (str) {
    var obj = utils.flag(this, 'object');
    if (obj instanceof Foo) {
      new chai.Assertion(obj.value).to.equal(str);
    } else {
      _super.apply(this, arguments);
    }
  }
});

Can also be accessed directly from chai.Assertion.

chai.Assertion.overwriteMethod('foo', fn);

Then can be used as any other assertion.

expect(myFoo).to.equal('bar');