Jest JS: What is jestjs? What is it for?

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Jest is an excellent JavaScript testing framework focused on simplicity.

It works with projects that use: Babel, TypeScript, Node, React, Angular, Vue, and much more!

To get started, you have to install it in your project with this command, depending on which installer you are using.

Introduction

Install Jest using your favorite package manager:

npm install --save-dev jest

yarn add --dev jest

pnpm add --save-dev jest

Let’s start by writing a test for a hypothetical function that adds two numbers. First, create a file named sum.js:

function sum(a, b) {
  return a + b;
}
module.exports = sum;

Next, create a file called sum.test.js. It will contain our actual test:

const sum = require('./sum');

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

Add the following section to your package.json:

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

Finally, run the yarn test or npm test, and Jest will display this message:

PASS  ./sum.test.js
✓ adds 1 + 2 to equal 3 (5ms)

You have just successfully written your first test using Jest!

This test used expect and toBe to test whether two values were exactly identical. To learn more about other things Jest can test, see Using Matchers.

Well done, friends! All this information is in the Jest doc’s getting-started guide. I just copied it here so you can read it quickly and get excited about studying like me ;-p. I’m already studying, how about you? Come with me and let’s study together. Knowledge doesn’t take up space, and we’ll be less dumb 🙂 For this type of article, it has to be very short, because people like me who always procrastinate are writing this article and it’s a victory.

Using Matchers

Jest uses “matchers” to let you test values in different ways. This document will introduce some commonly used matchers. For the full list, see the expect API doc.

Common Matchers

The simplest way to test a value is with exact equality.

test(‘dois mais dois é quatro’, () => {
  expect(2 + 2).toBe(4);
});

In this code, expect(2 + 2) returns an “expectation” object. You typically won’t do much with these expectation objects except call matchers on them. In this code, .toBe(4) is the matcher. When Jest runs, it tracks all the failing matchers so that it can print out nice error messages for you.

toBe uses Object.is to test exact equality. If you want to check the value of an object, use toEqual:

test(‘atribuição de objeto’, () => {
  const data = {one: 1};
  data[‘two’] = 2;
  expect(data).toEqual({one: 1, two: 2});
});

toEqual recursively checks every field of an object or array.

As promised at the beginning of the article, here are nine more libraries and their types of tests. No need to thank me, just sit down and start studying ;-p according to your needs.

Testing Library

• Component and Integration Testing (UI)
• https://testing-library.com

Mocha

• Unit and Integration Tests
• https://mochajs.org

Cypress

• End-to-End (E2E) Tests
• https://www.cypress.io

Playwright

• E2E Testing and Browser Automation
• https://playwright.dev

Puppeteer

• E2E Testing and Chrome/Chromium Automation
• https://pptr.dev

Vitest

• Unit Tests (Native Framework for Vite)
• https://vitest.dev

Jasmine

• Unit and Behavioral Tests (BDD)
• https://jasmine.github.io

Karma

• Tests in Real Browsers
• https://karma-runner.github.io

Selenium

• E2E Testing and Web Automation
• https://www.selenium.dev

Note

Please help me improve my articles. If you find any serious grammatical errors, I would be very grateful for your help. I am also studying English and use translators and AI to assist me, but there is nothing better than making a new friend and getting help from an expert.

Conclusion

In summary, adopting JEST is essential for a robust testing strategy in JavaScript/TypeScript. Its main advantage is its simplicity of configuration (zero-config for most projects), which speeds up the start of development. JEST offers a complete ecosystem, including a fast test runner, powerful mocking capabilities, built-in code coverage, and a runner for parallelized testing. This combination of ease of use, performance, and built-in features optimizes workflow, ensuring code reliability and a superior development experience (DX), as detailed in its documentation.

Sources

• https://jestjs.io
• https://jestjs.io/docs/getting-started
• https://413blocks.com