.addProperty(ctx, name, getter)

Adds a property to the prototype of an object.

utils.addProperty(chai.Assertion.prototype, 'foo', function () {
    var obj = utils.flag(this, 'object');
    new chai.Assertion(obj).to.be.instanceof(Foo);
});

Can also be accessed directly from chai.Assertion.

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

Then can be used as any other assertion.

expect(myFoo).to.be.foo;