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What's this?

This a synchronous implementation od JavaScript Promise. JestMockPromise implements the same API as native JavaScript Promise, with two added benefits:

  • it works synchronously - it calls the handler functions (then, catch, finally) right away (examples below clarify this)
  • it exposes the resolve and reject functions as instance methods allowig the promise to be resolved outside of executor function

Although its originally created to make writing unit tests easier, it can also be used outside the unit testing context.

What's in this document?


Why is it then called jest-mock-promise?

Because it was originally written as a part of the jest-mock-fetch project and still is it's integral part (as an external dependency). So it's name is a legacy of it's humble beginnings :)

Can it be used in unit testing?

The answer is Yes! Since it's not married with Jest it can also be used with other JavaScript testing frameworks (i.e. Mocha, Jasmine).


The API of this synchronous promise matches the one of the regular Promise, with two additional instance methods (attached to an instance of the Promise):

  • resolve - resolves a promise instance
  • reject - rejects a promise instance

This methods do the same job as the ones passed to the main callback function:

new Promise<string>((resolve, reject) => { resolve("some string value");  });

Having resolve and reject attached as instance methods enables us to call them outside the callback function, which makes our code much more readable:

let promise = new Promise<string>();

// resolving a promise
promise.resolve("some string value");

JestMockPromise vs native Promise

Let's jump right in and see an example:

import JestMockPromise from "../lib/jest-mock-promise";

const promise = new JestMockPromise((resolve, reject) => {
    // do you business normal here as normaly

console.log('1. Promise is ready');

promise.then(() => console.log('3. Promise is resolved');

console.log('2. Will resolve the promise');
promise.resolve(); // resolving the promise directly

console.log('4. Last line of code');

Let's now have a look at what the console output will look:

1. Promise is ready
2. Will resolve the promise
3. Promise is resolved
4. Last line of code

As you can see, the then handler was executed before the last line of code!

The crutial thing here is that this promise exposes the resolve function as an instance method, which means that you can call it directly on the instance of the promise object. This becomes escpecially useful in unit testing when you need to mock a component which returns a promise (i.e. jest-mock-fetch).

Let's now try to implement something similar using regular Promise object:

// here we'll store the resolve Function
let resolveFn;

// creating a new promise
let promise = new Promise((resolve, reject) => {
    // assigning the resolve function a variable from outter scope
    resolveFn = resolve;
    console.log('1. Promise is ready');

// attaching a `then` handler
promise.then(() => console.log('3. Promise is resolved'));

// resolving the promise right away
console.log('2. Will resolve the promise');

console.log('4. Last line of code');

The regular promise would produce the following console output:

1. Promise is ready
2. Will resolve the promise
4. Last line of code
3. Promise is resolved

If you compare this with the first example you can notice the following:

  1. the order of execution is different - "3. Promise is resolved" is logged to the console AFTER "4. Last line of code"
  2. in order to access the resolve function you need to store to an outside variable within the Promise callback function

An unit testing example

Synchronous Promise was created to simplify unit testing an async component. So in the next two examples we'll have a look at how we can do just that - simplify a unit test.

What we'll be testing

We are going to test a component, which multiplies two numbers provided as a payload of a promise. The result is returned a call to a callback function.

The following snippet shows implementation of that component:

// ./src/component.js
import Promise from 'es6-promise';

const onPromiseMultiply = (promise, callback) => {
    promise.then(([a,b]) => {

export {onPromiseMultiply};

Now let's write some Jest tests.

First example - Traditional async test

In our first example we'll create a test in a traditional async way ... just to show how terible it is. Then, in the second example, we'll improve on the original idea by introducing jest-mock-promise.

The next snippet contains a test written in traditional async way:

// ./src/__test__/component.spec.js
import {onPromiseMultiply} from '../component.js';
import Promise from 'jest-mock-promise';

describe('testing the multiply component', () => {

    it('should multiply two numbers and provide the result to the callback function', () => {

        let callbackFn = jest.fn();
        let promise = new Promise<[number,number]>((resolve, reject) {
            // providing two numbers which need to be multiplied
            // as we know, although we have resolved the promise right away,
            // `then` handlers will be called asnyc at later time

        // calling the function we want to test
        onPromiseMultiply(promise, callbackFn);

        // Altought promise is already resolved, `then` handlers will
        // be called async at later time. That's why we need to put
        // our expectation inside a `then` handler
        // + we need to return a promise to the Jest, so it knows
        // we're doing some async testing
        return(promise.then(() => {

As we can see, it's not easy to see the order in which our code gets executed. Can we make this better? Yes we can! In the following section we'll see how ...

Second example - Applying the synchronous Promise

The first thing we need to do is install this component: npm i --save-dev jest-mock-promise

Since our component uses es6-promise, we'll manually mock this dependency (if you don't know what manual mocking is, have a look at Manual Mocks @ Jest ). We'll create a __mocks__ directory inside our project root. There we'll create a es6-promise.js file with the following content:

// ./__mocks__/es6-promise.js
import JestMockPromise from 'jest-mock-promise';

// mocking the es6-promise, which is used by component we are testing
export { JestMockPromise as Promise };

Now that's set up, we can modify our test:

// ./src/__test__/component.spec.js
import {onPromiseMultiply} from '../component.js';
import Promise from 'jest-mock-promise';

describe('testing the multiply component', () => {

    it('should multiply two numbers and provide the result to the callback function', () => {

        let callbackFn = jest.fn();
        let promise = new Promise<[number,number]>();

        // calling the function we want to test
        onPromiseMultiply(promise, callbackFn);

        // resolving our promise

        // testing to see if our function is working correctly

As we can see, reading our code just became much easier! Hooray!

A working implementation of this example can be found in the example directory of this project.

Third example - Mocking fetch

As the final example we can have a look source code of jest-mock-fetch, which is based on jest-mock-promise.


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