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Unit Testing & TDD

Unit testing and test-driven development (TDD) are essential practices for developing maintainable and robust applications. In this tutorial, we'll introduce you to unit testing in TypeScript using Jest, a popular testing library.

Installing Jest and TypeScript Setup

To start, install Jest and its TypeScript dependencies:

npm install --save-dev jest ts-jest @types/jest typescript

Create a jest.config.js file in your project root and add the following configuration:

module.exports = {
preset: 'ts-jest',
testEnvironment: 'node',

Writing a Basic Test

Let's create a simple TypeScript function to test. Create a sum.ts file with the following code:

export function sum(a: number, b: number): number {
return a + b;

Now, create a test file named sum.test.ts:

import { sum } from './sum';

test('adds 1 + 2 to equal 3', () => {
expect(sum(1, 2)).toBe(3);

To run the test, add the following script to your package.json:

"scripts": {
"test": "jest"

Run the test using npm test.

Test-driven Development (TDD)

TDD is a development practice where you write tests before implementing the functionality. The process usually follows these steps:

  1. Write a failing test.
  2. Implement the code to make the test pass.
  3. Refactor the code, if necessary.

Let's use TDD to implement a subtract function. First, write a failing test in subtract.test.ts:

import { subtract } from './subtract';

test('subtracts 5 - 3 to equal 2', () => {
expect(subtract(5, 3)).toBe(2);

Run the test and see it fail. Now, implement the subtract function in subtract.ts:

export function subtract(a: number, b: number): number {
return a - b;

Run the test again, and it should pass. Refactor the code if needed.


In this tutorial, we've learned the basics of unit testing and test-driven development in TypeScript using Jest. By following these practices, you can improve the quality and maintainability of your code.