.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');