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Cédric Teyton for Promyze

Posted on • Updated on • Originally published at

Setup Mocha in watch mode for TDD in Node.js

Test-Driven Development is a great development (not testing) methodology. It definitively helps you to produce :

  • Minimal code to implement business needs
  • Small functions
  • Code covered by tests, avoiding later regressions

We use it every day at Promyze and we definitively saw the benefits of coding in this way.


Now, let's see how you can apply TDD with Mocha with Typescript on a Node.js project.

All the code is available on this Github project.

Code Structure

In this example we have this simple structure :

├── domains
│   └── Bowling
│       ├── Game.spec.ts
│       └── Game.ts
├── package.json
├── package-lock.json
└── test
    └── mocha.spec.ts
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domains contains our source code, and test contains my main test suite.

Let's have a look at the code now.


The interesting part here is the scripts section. As you can see, we added the --watch option of Mocha in addition with the --watch-files option. Here, all the code in domain and test folders will be checked for modification, and tests will be automatically run again.

    "name": "node-tu-wacher",
    "devDependencies": {
        "@types/chai": "^4.2.18",
        "@types/mocha": "^8.2.2",
        "@types/node": "^15.6.2",
        "chai": "^4.3.4",
        "mocha": "^8.4.0",
        "ts-node": "^10.0.0",
        "typescript": "^4.3.2"
    "engines": {
        "node": ">=14.16.1"
    "scripts": {
        "test": " mocha -r ts-node/register test/mocha.spec.ts",
        "test:watch": " mocha -r ts-node/register --watch --watch-files domains/**/*.ts,test/*.ts test/mocha.spec.ts"
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This is the root test file that will import all my test suites :

import GameSpec from '../domains/Bowling/Game.spec';

import { Suite } from 'mocha';

describe('Server unit testing', function(this: Suite) {
        describe('Game', GameSpec);
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Here we assume that I've just started to code a Bowling app. I wrote my first test :

import { expect } from 'chai';
import Game from './Game';

export default function() {
    describe('Game testing', function() {

        it('should start a bowling game', async function() {
            const game = new Game();

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And here a basic implementation so that my first test can run :

export default class Game {

    private score: Number;

    constructor() {
        this.score = 0;

    start(): void {
        this.score = 0;

    currentScore(): Number {
        return this.score;

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See it in action

The command npm run test:watch will help me for my TDD session. Everytime a file is saved (CTRL+S with in your favorite IDE), tests would be run again.

cteyton@laptop:~/node-tu-watcher$ npm run test:watch

> node-tu-wacher@1.0.0 test:watch /home/cteyton/Work/Promyze/blog/node-tu-watcher
>  mocha -r ts-node/register --watch --watch-files domains/**/*.ts,test/*.ts test/mocha.spec.ts

  Server unit testing
      Game testing
        ✓ should start a bowling game

  1 passing (2ms)

ℹ [mocha] waiting for changes...
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Get started now and have fun with TDD ;-)

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