Javascript unit tests : 'Injecting mocks' with Jest

· 2 min · dev Javascript Tests

Table of contents

Purpose of this post

With a module A requiring a module B
When I want to test A
Show how to replace B by a mock using JestJS

Context

I am a Java developper. I am a heavy user of Mockito.
Often, I need to test a service and replace external systems, injected into my service, by Mocks.
I use “Inject Mocks”.

I need to do this in Javascript.
As I write “lambdas” for AWS.
I need to test them and avoid calling S3, SNS, DynamoDB, …

Let’s say we have a “Father” and a “Son”.
“Son” is required by “Father”

Simple test : everything is synchronous

father.js
'use strict';
const son = require('./son');
exports.saysWithSon = () => {
    return "-- Father : I am your father " + son.says();
};
son.js
'use strict';

exports.says = () => {
    return "-- Son : Nooo";
};
father.test.js
'use strict';

const father = require('./father');

// We require the son module
const son = require('./son');

// IMPORTANT : We ask jest to mock the son module
jest.mock('../son');

describe('Testing Father with Son', () => {

    test('should say', () => {
        // Given
        // We replace son.says by another implementation
        son.says.mockImplementation( () => "XXXX");
        // When
        let say = father.saysWithSon();
        // Then
        expect(say).toBe("-- Father : I am your father XXXX");
        // instead of 
        // "-- Father : I am your father -- Son : Nooo" 
        // without mocks
    });

});

K

asynchronous test with callbacks

Mocking a function with a callback is easy

father.js
'use strict';

const son = require('./son');

exports.saysToSon = ( callback ) => {
    son.reply( (sonsaid) => callback("-- Father : I am your father " + sonsaid));
};
son.js
'use strict';

exports.reply = (callback) => {
    callback("-- I say : Noo");
};
father.test.js
'use strict';

const tested = require('./father');
const son = require('./son');

jest.mock('./son');

describe('Testing Father with Son', () => {

    test('should say', (done) => {
        // Given
        son.reply.mockImplementation( (callback) => { callback("XXXX"); });
        // Then
        function callback(data) {
            expect(data).toBe("-- Father : I am your father XXXX");
            done();
        }
        // When
        tested.saysToSon(callback);
    });

});

Reference

TODO

I should update this post to add Promises