Sinon–Chai provides a set of custom assertions for using the Sinon.JS spy, stub, and mocking framework with the Chai assertion library. You get all the benefits of Chai with all the powerful tools of Sinon.JS.

Instead of using Sinon.JS's assertions:

sinon.assert.calledWith(mySpy, "foo");

or awkwardly trying to use Chai's should or expect interfaces on spy properties:

mySpy.calledWith("foo").should.be.ok;
expect(mySpy.calledWith("foo")).to.be.ok;

you can say

mySpy.should.have.been.calledWith("foo");
expect(mySpy).to.have.been.calledWith("foo");

All of your favorite Sinon.JS assertions made their way into Sinon–Chai. We show the should syntax here; the expect equivalent is also available.

For more information on the behavior of each assertion, see the documentation for the corresponding spy methods. These of course work on not only spies, but individual spy calls, stubs, and mocks as well.

Note that you can negate any assertion with Chai's .not. E. g. for notCalled use spy.should.have.not.been.called. Similarly, note that the always methods are accessed with Chai's .always prefix; should.have.been.alwaysCalledWith will not work - instead, use should.always.have.been.calledWith.

For simplicity, this library intentionally only implements Sinon's spy methods, and does not add an interface for Sinon.assert.match. Sinon's matchers are implemented by the samsam library, so if you want a should/expect interface to assert.match you may be interested in chai-samsam, which adds a .deep.match verb that will work with Sinon matchers.

For assert interface there is no need for sinon-chai or chai-samsam. You can install Sinon.JS assertions right into Chai's assert object with expose:

import * as chai from "chai";
import sinon from "sinon";

sinon.assert.expose(chai.assert, { prefix: "" });

Using Chai's should:

"use strict";
import * as chai from "chai";
import sinon from "sinon";
import sinonChai from "sinon-chai";
chai.should();
chai.use(sinonChai);

function hello(name, cb) {
    cb("hello " + name);
}

describe("hello", function () {
    it("should call callback with correct greeting", function () {
        var cb = sinon.spy();

        hello("foo", cb);

        cb.should.have.been.calledWith("hello foo");
    });
});

Using Chai's expect:

"use strict";
import * as chai from "chai";
import sinon from "sinon";
import sinonChai from "sinon-chai";
const expect = chai.expect;
chai.use(sinonChai);

function hello(name, cb) {
    cb("hello " + name);
}

describe("hello", function () {
    it("should call callback with correct greeting", function () {
        var cb = sinon.spy();

        hello("foo", cb);

        expect(cb).to.have.been.calledWith("hello foo");
    });
});

Do an npm install --save-dev sinon-chai to get up and running. Then:

import * as chai from "chai";
import sinonChai from "sinon-chai";

chai.use(sinonChai);

You can of course put this code in a common test fixture file; for an example using Mocha, see the Sinon–Chai tests themselves.

Thanks to Cymen Vig, there's now a Ruby gem of Sinon–Chai that integrates it with the Rails asset pipeline!