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

add.js

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

jest.mock('./calculator');  

import {jest} from '@jest/globals'

jest.mock('./calculator');  

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');
  addSpy.mockImplementation();  
  expect(addSpy).toHaveBeenCalled();  
});