Unit Testing with Typescript and Jest

November 06, 2018

In this article we'll try to cover a simple unit testing in Typescript + jest.

We're going to create a simple utility that detect whether an url is internal link or external link. For example https://www.google.com is an external link, while /page1 is an internal link. We're going to name the project is-internal-link, but you can name it anything.


  • NodeJS
  • VSCode + Jest plugin (Optional)

Create new directory

mkdir is-internal-link

Init npm

npm init

Install dependencies

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

Create jest.config.js

module.exports = {
  roots: ['<rootDir>/src'],
  transform: {
    '^.+\\.tsx?$': 'ts-jest',
  testRegex: '(/__tests__/.*|(\\.|/)(test|spec))\\.tsx?$',
  moduleFileExtensions: ['ts', 'tsx', 'js', 'jsx', 'json', 'node'],

Create tsconfig.json

  "compilerOptions": {
    "target": "es5",
    "module": "commonjs",
    "lib": ["es2015"],
    "strict": true,
    "declaration": true,
    "outDir": "dist",
    "sourceMap": true
  "include": ["src/**/*"],
  "exclude": ["node_modules", "**/*.spec.ts"]

All right we're ready to write the code. Create file src/main.ts and src/main.spec.ts Your files tree should now looks like this

├── node_modules
├── package-lock.json
├── package.json
├── src
│   ├── main.spec.ts
│   └── main.ts
└── tsconfig.json

Open it in your favorite editor. (VSCode / Atom / Sublime / etc). I personally use VSCode

import { isInternalLink } from './main'

test('should return false given external link', () => {

test('should return true given internal link', () => {

Now there's to way to test this.

Way 1

Open your package.json. And replace it your scripts with

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

Run npm run jest. Now you should see the error because we haven't implemented the code yet right?

Way 2

With your editor's plugin. I prefer this way. I'll only show it on VSCode. Your favorite editor might have it too.

Install vscode-jest

vscode-jest This GIF, should be self a great explanation how vscode-jest works

Let's continue. vscode-jest

Your VSCode might now looks like this.

Implementing the code

Let's implement our main.ts

export const isInternalLink = (link: string) => /^\/(?!\/)/.test(link)

Switch back to your main.spec.ts. You should see now the error is gone, and it turns green.

PRO TIPS: Use VSCode split editor to see the code (main.ts) and spec (main.spec.ts) at the same time.


Open main.ts on first side cmd+1 cmd+p main.ts

Open main.spec.ts on second side cmd+2 cmd+p1 main.spec.ts


You just did your Unit Testing with Typescript and Jest