.addMethod(ctx, name, method)

Adds a method to the prototype of an object.

utils.addMethod(chai.Assertion.prototype, 'foo', function (str) {
    var obj = utils.flag(this, 'object');
    new chai.Assertion(obj).to.be.equal(str);
});

Can also be accessed directly from chai.Assertion.

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

Then can be used as any other assertion.

expect(fooStr).to.be.foo('bar');