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Sean Kerr

Full Stack Engineer

What is the unit test mocking?

Mocking is generally used in unit testing. Sometimes, you may have complex scenarios that need to be tested in isolation. You can simulate the behaviour of other units by mocking their functionality.


I'm using jest and Typescript for my demonstrations. The code for this article can be found here.

So why did I choose to do this in Typescript? Well, I was working in an organisation that was using Typescript for most of its codebase. I started to see some tests that had been written in javascript. I began to wonder why this was happening. As it turned out, some of the developers were having issues using Typescript in their tests.

If you're not using the code in this article, and you wish to create your own project, to do this in then you will need to follow the steps below.

create a new folder

mkdir jest-testing

Initialise a new project

npm init

Follow all of the prompts until you have a basic project with a package.json file in it.

Then install the following packages

yarn add typescript @types/jest jest ts-jest @jest/globals

Now you can create your Typescript files for your functions and classes

This is why I felt compelled to write this article. So without further ado lets jump right in.

Mocking a function

Mocking a Typescript function. Consider the following:

export const add = (): number => {
  1 + 2;

In our test we need to import the function that we want to test

import * as a from "./add";

Then we need to mock our function

jest.mock("./add", () => ({
  add: jest.fn(),

At this point I'm thinking we are all ready to run our test but, there is one thing missing. We need to cast it as a type of jest.Mock.

const mockAdd = a.add as jest.Mock;

Okay so we have everything in place lets test our function.

First we will create a test group using describe, then we will add our actuall test. I've used it here but, you also can use test. Next, I want to mock what my function returns so I use the built in jest function called mockReturnValue. In this case, my function is returning a number so I'm telling it to return 3. Then I'm going to call the actual function and my expectation is that the function will return the value of 3, due to the fact we have used Jest to mock our return value.

describe("add function", () => {
  it("should pass", () => {
    const res = a.add();

So the test will pass as we are getting the value of 3 from our function. Great! Thats the job done right? Not so fast.

Say we have a Typescript class? That class has some methods and we want to test those methods? This is a contrived example to illustrate.

interface User {}
interface TheUser {
  getSalary(id: string): Promise<User>;
interface SomeClientOfSomething {
  request(method: string, endpoint: string): Promise<{ data: any }>;
export default class Employee implements TheUser {
  constructor(private readonly someClient: SomeClientOfSomething) {}

  public async getSalary(id: string): Promise<User> {
    const response = await this.someClient.request(


So I want to test the getSalary method of that class. How can I do this?

First, I create the test file and import my class like

import Employee from "./Employee";

Then I create my descibe block and then create my test.

describe("some class", () => {
  it("should pass", async () => {
      //test code here

Note the async keyword as part of the test. We are testing an asynchronous function so we need to ensure that test is async or we won't get the result we desire.

Now, inside my test I start to do the following. Create my output or what I'm expecting to return from my test.

const theResponseIExpect = { salary: 2000 };

Then I mock the request

const SomeClientOfSomething = {
  request: jest.fn().mockResolvedValueOnce({ data: theResponseIExpect }),

Then we need initialise our class and call our function

const theSomeTest = new Employee(SomeClientOfSomething);
const actual = await theSomeTest.getSalary("12");

Now we can assert on the response and assert on the call itself.


Ok so we have learned a little about mocking with Jest. There's one last thing I would like to cover. As an example, say I want to use that class in one of my functions and then test the function without testing the class method? In other words, I'm using that method in my class but I just want to mock its behaviour.

Well, you could use an automatic mock like


And then you could get access to the class like

const mockEmployeeInstance = Employee.mock.instances[0];
const mockMethod1 = mockEmployeeInstance.getSalary;

This is handy when you need to test a method that uses a class method in its implementation. Of course, you don't need to test the class method, only mock it.

I hope you enjoyed the article which is a small introduction to the mocking with Jest. There's plenty of information that I didn't cover here. You can read more about Jest here

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