# Chai Assertions for Promises **Chai as Promised** extends [Chai][chai] with a fluent language for asserting facts about [promises][presentation]. Instead of manually wiring up your expectations to a promise's fulfilled and rejected handlers: ```javascript doSomethingAsync().then( function (result) { result.should.equal("foo"); done(); }, function (err) { done(err); } ); ``` you can write code that expresses what you really mean: ```javascript return doSomethingAsync().should.eventually.equal("foo"); ``` or if you have a case where `return` is not preferable (e.g. style considerations) or not possible (e.g. the testing framework doesn't allow returning promises to signal asynchronous test completion), then you can use the following workaround (where `done()` is supplied by the test framework): ```javascript doSomethingAsync().should.eventually.equal("foo").notify(done); ``` *Notice*: either `return` or `notify(done)` _must_ be used with promise assertions. This can be a slight departure from the existing format of assertions being used on a project or by a team. Those other assertions are likely synchronous and thus do not require special handling. ## How to Use ### `should`/`expect` Interface The most powerful extension provided by Chai as Promised is the `eventually` property. With it, you can transform any existing Chai assertion into one that acts on a promise: ```javascript (2 + 2).should.equal(4); // becomes return Promise.resolve(2 + 2).should.eventually.equal(4); expect({ foo: "bar" }).to.have.property("foo"); // becomes return expect(Promise.resolve({ foo: "bar" })).to.eventually.have.property("foo"); ``` There are also a few promise-specific extensions (with the usual `expect` equivalents also available): ```javascript return promise.should.be.fulfilled; return promise.should.eventually.deep.equal("foo"); return promise.should.become("foo"); // same as `.eventually.deep.equal` return promise.should.be.rejected; return promise.should.be.rejectedWith(Error); // other variants of Chai's `throw` assertion work too. ``` ### `assert` Interface As with the `should`/`expect` interface, Chai as Promised provides an `eventually` extender to `chai.assert`, allowing any existing Chai assertion to be used on a promise: ```javascript assert.equal(2 + 2, 4, "This had better be true"); // becomes return assert.eventually.equal(Promise.resolve(2 + 2), 4, "This had better be true, eventually"); ``` And there are, of course, promise-specific extensions: ```javascript return assert.isFulfilled(promise, "optional message"); return assert.becomes(promise, "foo", "optional message"); return assert.doesNotBecome(promise, "foo", "optional message"); return assert.isRejected(promise, "optional message"); return assert.isRejected(promise, Error, "optional message"); return assert.isRejected(promise, /error message matcher/, "optional message"); ``` ### Progress Callbacks Chai as Promised does not have any intrinsic support for testing promise progress callbacks. The properties you would want to test are probably much better suited to a library like [Sinon.JS][sinon], perhaps in conjunction with [Sinon–Chai][sinon-chai]: ```javascript var progressSpy = sinon.spy(); return promise.then(null, null, progressSpy).then(function () { progressSpy.should.have.been.calledWith("33%"); progressSpy.should.have.been.calledWith("67%"); progressSpy.should.have.been.calledThrice; }); ``` ### Customizing Output Promises By default, the promises returned by Chai as Promised's assertions are regular Chai assertion objects, extended with a single `then` method derived from the input promise. To change this behavior, for instance to output a promise with more useful sugar methods such as are found in most promise libraries, you can override `chaiAsPromised.transferPromiseness`. Here's an example that transfer's Q's `finally` and `done` methods: ```js chaiAsPromised.transferPromiseness = function (assertion, promise) { assertion.then = promise.then.bind(promise); // this is all you get by default assertion.finally = promise.finally.bind(promise); assertion.done = promise.done.bind(promise); }; ``` ### Transforming Arguments to the Asserters Another advanced customization hook Chai as Promised allows is if you want to transform the arguments to the asserters, possibly asynchronously. Here is a toy example: ```js chaiAsPromised.transformAsserterArgs = function (args) { return args.map(function (x) { return x + 1; }); } Promise.resolve(2).should.eventually.equal(2); // will now fail! Promise.resolve(2).should.eventually.equal(3); // will now pass! ``` The transform can even be asynchronous, returning a promise for an array instead of an array directly. An example of that might be using `Promise.all` so that an array of promises becomes a promise for an array. If you do that, then you can compare promises against other promises using the asserters: ```js // This will normally fail, since within() only works on numbers. Promise.resolve(2).should.eventually.be.within(Promise.resolve(1), Promise.resolve(6)); chaiAsPromised.transformAsserterArgs = function (args) { return Promise.all(args); }; // But now it will pass, since we transformed the array of promises for numbers into // (a promise for) an array of numbers Promise.resolve(2).should.eventually.be.within(Promise.resolve(1), Promise.resolve(6)); ``` ### Compatibility Chai as Promised is compatible with all promises following the [Promises/A+ specification][spec]. Notably, jQuery's so-called “promises” are not up to spec, and Chai as Promised will not work with them. In particular, Chai as Promised makes extensive use of the standard [transformation behavior][] of `then`, which jQuery does not support. ### Working with Non-Promise–Friendly Test Runners Some test runners (e.g. Jasmine, QUnit, or tap/tape) do not have the ability to use the returned promise to signal asynchronous test completion. If possible, I'd recommend switching to ones that do, such as [Mocha][mocha-promises], [Buster][buster-promises], or [blue-tape][]. But if that's not an option, Chai as Promised still has you covered. As long as your test framework takes a callback indicating when the asynchronous test run is over, Chai as Promised can adapt to that situation with its `notify` method, like so: ```javascript it("should be fulfilled", function (done) { promise.should.be.fulfilled.and.notify(done); }); it("should be rejected", function (done) { otherPromise.should.be.rejected.and.notify(done); }); ``` In these examples, if the conditions are not met, the test runner will receive an error of the form `"expected promise to be fulfilled but it was rejected with [Error: error message]"`, or `"expected promise to be rejected but it was fulfilled."` There's another form of `notify` which is useful in certain situations, like doing assertions after a promise is complete. For example: ```javascript it("should change the state", function (done) { otherState.should.equal("before"); promise.should.be.fulfilled.then(function () { otherState.should.equal("after"); }).should.notify(done); }); ``` Notice how `.notify(done)` is hanging directly off of `.should`, instead of appearing after a promise assertion. This indicates to Chai as Promised that it should pass fulfillment or rejection directly through to the testing framework. Thus, the above code will fail with a Chai as Promised error (`"expected promise to be fulfilled…"`) if `promise` is rejected, but will fail with a simple Chai error (`expected "before" to equal "after"`) if `otherState` does not change. ### Multiple Promise Assertions To perform assertions on multiple promises, use `Promise.all` to combine multiple Chai as Promised assertions: ```javascript it("should all be well", function () { return Promise.all([ promiseA.should.become("happy"), promiseB.should.eventually.have.property("fun times"), promiseC.should.be.rejectedWith(TypeError, "only joyful types are allowed") ]); }); ``` This will pass any failures of the individual promise assertions up to the test framework, instead of wrapping them in an `"expected promise to be fulfilled…"` message as would happen if you did `return Promise.all([…]).should.be.fulfilled`. If you can't use `return`, then use `.should.notify(done)`, similar to the previous examples. ## Installation and Setup ### Node Do an `npm install chai-as-promised` to get up and running. Then: ```javascript var chai = require("chai"); var chaiAsPromised = require("chai-as-promised"); chai.use(chaiAsPromised); ``` You can of course put this code in a common test fixture file; for an example using [Mocha][], see [the Chai as Promised tests themselves][fixturedemo]. ### AMD Chai as Promised supports being used as an [AMD][amd] module, registering itself anonymously (just like Chai). So, assuming you have configured your loader to map the Chai and Chai as Promised files to the respective module IDs `"chai"` and `"chai-as-promised"`, you can use them as follows: ```javascript define(function (require, exports, module) { var chai = require("chai"); var chaiAsPromised = require("chai-as-promised"); chai.use(chaiAsPromised); }); ``` ### ` ``` ### Karma If you're using [Karma][], check out the accompanying [karma-chai-as-promised][] plugin. ### Browser Compatibility Chai as Promised is only compatible with modern browsers (IE ≥9, Safari ≥6, no PhantomJS). [presentation]: http://www.slideshare.net/domenicdenicola/callbacks-promises-and-coroutines-oh-my-the-evolution-of-asynchronicity-in-javascript [chai]: http://chaijs.com/ [Mocha-promises]: http://mochajs.org/#asynchronous-code [Buster-promises]: http://docs.busterjs.org/en/latest/modules/buster-test/spec/#returning-a-promise [blue-tape]: https://github.com/spion/blue-tape [spec]: http://promisesaplus.com/ [transformation behavior]: http://domenic.me/2012/10/14/youre-missing-the-point-of-promises/#toc_2 [Mocha]: http://mochajs.org [fixturedemo]: https://github.com/domenic/chai-as-promised/tree/master/test/ [amd]: https://github.com/amdjs/amdjs-api/wiki/AMD [sinon]: http://sinonjs.org/ [sinon-chai]: https://github.com/domenic/sinon-chai [Karma]: https://karma-runner.github.io/ [karma-chai-as-promised]: https://github.com/vlkosinov/karma-chai-as-promised