Jest How to test modules and single functions with examples

Jest is a testing framework to test different javascript components.

This tutorial explains how to test and mock entire modules and single functions

First, let’s define a single function


export const add = (a, b) => a + b;

Let’s define a Module

import { add } from "./add";

export const sum = (a, b) => add(a + b);

How to Mock entire modules in javascript using Jest

the calculator is a module that calls other named functions from other files.

To mock the entire module automatically, use jest.mock() function

import { jest } from "@jest/globals";


import { add } from "./add";
import { sum } from "./calculator";

test("calculator tests", () => {
  const value = sum(1, 2);
  expect(value).toEqual(3); // Success!

How to Mock single function from modules in javascript using Jest

To mock a single function First, Create a spy of the function using jest.spyon call spy.mockImplementation() method.

test the method is called using the toHaveBeenCalled method

Here is an example test case file

import { jest } from "@jest/globals";

import * as addObj from "./add";
import { sum } from "./calculator";

test("calculator test single test", () => {
  const addSpy = jest.spyOn(addObj, "add");