Jest

The long-standing JavaScript test runner, with mocking and snapshots included.

What it is

Jest is a delightful JavaScript testing framework with a focus on simplicity, correctness, and developer experience. It supports unit, integration, and snapshot testing for JavaScript and TypeScript projects.

Jest allows developers to write tests using simple syntax and run them in a fast and isolated environment. It supports features like mocks, spies, snapshot testing, code coverage, and parallel test execution.

Licence: MIT
Watch for: Snapshot tests are easy to update thoughtlessly and stop catching anything

When to use it

The question documentation cannot answer for you — because it cannot recommend something else.

Reach for it when

Existing projects already configured for it
React Native, where it is the standard

Look elsewhere when

New Vite projects — Vitest is faster and needs almost no configuration
ESM-heavy codebases, where Jest's module handling still causes friction

Installation

npm install --save-dev jest

Getting started

The smallest useful thing you can do with it, and what each part means.

Basic test example

javascript

1234567function sum(a, b) {
  return a + b;
}

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

Defines a simple unit test that verifies the sum function returns the correct result.

Using beforeEach

javascript

1234567let counter = 0;
beforeEach(() => {
  counter = 0;
});
test('counter starts at 0', () => {
  expect(counter).toBe(0);
});

Resets the counter before each test using `beforeEach`.

Advanced usage

Where the library earns its place over a simpler alternative.

Mocking functions

javascript

const myFunc = jest.fn();
myFunc('hello');
expect(myFunc).toHaveBeenCalledWith('hello');

Demonstrates using Jest's `fn()` to create a mock function and assert it was called with specific arguments.

Snapshot testing

javascript

1234const user = { name: 'Alice', age: 25 };
test('user snapshot', () => {
  expect(user).toMatchSnapshot();
});

Uses snapshot testing to verify that the object structure does not change unexpectedly.

Testing asynchronous code

javascript

1234567function fetchData() {
  return Promise.resolve('peanut butter');
}
test('the data is peanut butter', async () => {
  const data = await fetchData();
  expect(data).toBe('peanut butter');
});

Tests an asynchronous function using `async/await` syntax.

Mocking modules

javascript

jest.mock('./api');
import { fetchData } from './api';
fetchData.mockResolvedValue('mocked data');

Shows how to mock entire modules to control dependencies in tests.

Errors and fixes

The failures you are most likely to hit, and what actually resolves them.

Test failed due to unexpected value: Check the expected value, and ensure the function under test produces consistent outputs.
Cannot find module: Verify module paths and use `jest.mock()` appropriately when mocking dependencies.
Timeout - Async callback was not invoked: Ensure asynchronous tests call `done()` or return a promise, and adjust timeout if necessary.

Best practices

Write small, focused tests for individual units of code.
Use mocks and spies to isolate the code under test.
Leverage snapshot testing for UI components and serializable objects.
Run tests in watch mode during development for faster feedback.
Organize test files alongside source files or in a __tests__ directory for clarity.

Alternatives

Comparable options, and the reason you would pick one over the other.

Vitest
Jest-compatible API, natively ESM, much faster; the default for new projects

Background

Why it exists, and what it was reacting to.

Jest was created by Facebook to test React applications, but it has grown to support any JavaScript project. Its zero-configuration setup, powerful mocking capabilities, and snapshot testing features make it widely popular for testing modern web applications.

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Jest

Reach for it when

Look elsewhere when

Vitest

Vitest

Cypress

Playwright