Jest — JavaScript Testing Framework

Jest brings a batteries-included approach to JavaScript testing. Where other frameworks require you to assemble a test runner, assertion library, and mocking tool separately, Jest ships all three in a single package. You install it, write a test, and run it. That simplicity is why it dominates the JavaScript testing landscape.

This skill walks you through Jest from first principles — writing assertions, mocking dependencies, testing asynchronous code, generating coverage reports, and integrating everything into a CI pipeline.

Installing and Configuring Jest

Every Jest setup begins with installation and a configuration file. Jest works out of the box for plain JavaScript, but most real projects need a bit of configuration for TypeScript, JSX, or module resolution.

# Install Jest and its TypeScript support
npm install --save-dev jest ts-jest @types/jest

Once installed, create a configuration file at the root of your project. The jest.config.ts format gives you type checking on your configuration options.

// jest.config.ts — Root Jest configuration for a TypeScript project