chai-as-promised

What is chai-as-promised?

The chai-as-promised npm package extends Chai with a fluent language for asserting facts about promises. It allows developers to work with promises in their test assertions in a more natural and expressive way, which is particularly useful when dealing with asynchronous operations in tests.

What are chai-as-promised's main functionalities?

Asserting a promise is fulfilled

This feature allows you to assert that a promise will be fulfilled. In other words, it checks that the promise will eventually resolve.

expect(promise).to.be.fulfilled;

Asserting a promise is rejected

This feature allows you to assert that a promise will be rejected. This is useful for testing error handling in asynchronous code.

expect(promise).to.be.rejected;

Asserting a promise is rejected with a specific error

With this feature, you can assert that a promise is not only rejected but also that it is rejected with a specific type of error.

expect(promise).to.be.rejectedWith(Error);

Asserting a promise's fulfillment value

This feature allows you to assert what value a promise will be resolved with. The 'eventually' chain is used to wait for the promise to resolve before making the assertion.

expect(promise).to.eventually.equal('foo');

Chaining assertions

chai-as-promised supports chaining assertions, allowing for more complex assertions on the resolved value of a promise.

expect(promise).to.eventually.have.property('bar').that.is.a('string');

Other packages similar to chai-as-promised

jest

Jest is a testing framework that includes its own assertion library and has built-in support for testing asynchronous code. It can handle promises and async/await syntax without the need for additional plugins.

expect

Expect is an assertion library that can be used independently or with Jest. It provides a way to make assertions about promises, similar to chai-as-promised, but it is more tightly integrated with Jest when used in that context.

README

Chai Assertions for Promises

Chai as Promised extends Chai with a fluent language for asserting facts about promises.

Instead of manually wiring up your expectations to a promise's fulfilled and rejected handlers:

doSomethingAsync().then(
    function (result) {
        result.should.equal("foo");
        done();
    },
    function (err) {
       done(err);
    }
);

you can write code that expresses what you really mean:

return doSomethingAsync().should.eventually.equal("foo");

or if you have a testing framework that doesn't allow returning promises to signal asynchronous test completion, then you can use the following workaround:

doSomethingAsync().should.eventually.equal("foo").notify(done);

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:

(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):

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:

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:

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, perhaps in conjunction with Sinon–Chai:

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:

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:

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:

// 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. 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, Buster, 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:

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:

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.

Another example of where this can be useful is when performing assertions on multiple promises:

it("should all be well", function (done) {
    Q.all([
        promiseA.should.become("happy"),
        promiseB.should.eventually.have.property("fun times"),
        promiseC.should.be.rejectedWith(TypeError, "only joyful types are allowed")
    ]).should.notify(done);
});

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 Q.all([…]).should.be.fulfilled.and.notify(done).

Installation and Setup

Node

Do an npm install chai-as-promised to get up and running. Then:

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.

AMD

Chai as Promised supports being used as an 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:

define(function (require, exports, module) {
    var chai = require("chai");
    var chaiAsPromised = require("chai-as-promised");

    chai.use(chaiAsPromised);
});

<script> tag

If you include Chai as Promised directly with a <script> tag, after the one for Chai itself, then it will automatically plug in to Chai and be ready for use:

<script src="chai.js"></script>
<script src="chai-as-promised.js"></script>

Browser Compatibility

Chai as Promised is only compatible with modern browsers (IE ≥9, Safari ≥6, no PhantomJS).