JavaScript Testing: Where to Start for Beginners in 2024

Why JavaScript Testing Is Not Scary, But Useful?

When I first started writing in JavaScript, I thought testing was something akin to high magic. "Why? I can see that the code works!" — I thought. And then Monday came, I added one small function, and everything broke. Sound familiar?

JavaScript testing is like insurance for your code. You write it once, and you check its work hundreds of times automatically. And believe me, when your project grows to 10,000 lines of code, you will thank the person who made you write your first test.

In this article, I will tell you where to start JavaScript testing for a beginner, show real examples, and provide ready-made templates. You will be surprised, but you can write your first tests in just 15 minutes.

Types of Tests: What a Beginner Needs to Know

Before writing code, let's understand the theory. There are three main levels of testing, but to start, we only need one.

Unit tests

These are tests for the smallest pieces of code — functions, methods, classes. They check: "Does this particular function work the way I intended?". Unit tests JavaScript are the foundation everyone starts with.

Integration tests

They check how different parts of the system work together. For example, does the "form → data submission → server response" chain work.

E2E tests (End-to-End)

The most complex. They simulate real user actions: clicks, form filling, page navigation. For a beginner, this is the next level.

To start, it is enough to master unit tests. Everything else will come with experience.

Tools: What to Choose for a Beginner?

The market for JavaScript testing tools is huge. But I will highlight three combinations that are actually usable for beginners:

• Jest — the most popular framework. Works "out of the box", requires no complex setup. Ideal for starting.
• Mocha + Chai — a classic. Mocha runs tests, Chai provides convenient functions for assertions.
• Vitest — modern and fast, almost like Jest, but for projects on Vite.

I recommend starting with Jest. It is intuitive, has excellent documentation, and a huge community. If you master Jest, switching to other tools will be easy.

Writing Your First Test: Step-by-Step Guide

Let's write real code. Suppose we have a simple function that adds two numbers:

// math.jsfunction add(a, b) {  return a + b;}

module.exports = add;

Now let's install Jest. Open the terminal in your project folder and run:

npm init -ynpm install --save-dev jest

Add a script to package.json to run the tests:

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

Create a file math.test.js in the same folder. Let's write a test:

// math.test.jsconst add = require('./math');

test('adds 2 + 3 and gets 5', () => {  expect(add(2, 3)).toBe(5);});

test('adds negative numbers', () => {  expect(add(-1, -1)).toBe(-2);});

test('adds zero', () => {  expect(add(0, 0)).toBe(0);});

Run the tests with the command npm test. If everything is done correctly, you will see green checkmarks. Congratulations, you have just written your first JavaScript test!

How to Test More Complex Functions?

Okay, we've learned how to add numbers. But what if the function works with arrays or objects? Let's look at a more complex example.

A function for filtering users by age:

// users.jsfunction filterAdults(users) {  return users.filter(user => user.age >= 18)