Jest Setup

Installing Jest

Jest is distributed as an npm package. In any Node.js project, install it as a dev dependency, since tests only run during development and CI, never in production.

For plain CommonJS Node.js projects (using require/module.exports), no extra configuration is required. For projects using ES module import/export syntax or JSX, you typically add Babel so Jest can transform the code before running it.

npm install --save-dev jest

// package.json
{
  "scripts": {
    "test": "jest"
  }
}

After installing, npm test runs the full Jest suite using default configuration.

Babel Setup for import/export Syntax

npm install --save-dev babel-jest @babel/core @babel/preset-env

// babel.config.js
module.exports = {
  presets: [['@babel/preset-env', { targets: { node: 'current' } }]],
};

• babel-jest lets Jest transform files through Babel before executing tests.
• @babel/preset-env compiles modern syntax like import/export to CommonJS for Node.js.
• For React, also add @babel/preset-react; for TypeScript, add @babel/preset-typescript or use ts-jest.
• No jest.config.js is required for a minimal setup — Babel config alone is often enough.

Jest Setup Cheat Sheet

Common installation commands for different project types.

Setting Up Jest with TypeScript

For TypeScript projects, ts-jest is the most common transformer. It type-checks and compiles .ts files on the fly so Jest can run them directly, and it integrates with your existing tsconfig.json.

npm install --save-dev jest ts-jest @types/jest typescript
npx ts-jest config:init

// jest.config.js (generated)
module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node',
};

npx ts-jest config:init scaffolds a working jest.config.js automatically.

Useful npm Scripts

A small set of consistent npm scripts saves everyone on a team from memorizing raw Jest flags.

// package.json
{
  "scripts": {
    "test": "jest",
    "test:watch": "jest --watch",
    "test:coverage": "jest --coverage"
  }
}

Common Mistakes

• Installing Jest as a regular dependency instead of a dev dependency.
• Forgetting Babel configuration and getting "Unexpected token 'export'" errors.
• Mixing ts-jest and Babel-based TypeScript transforms in the same project.
• Not committing jest.config.js or babel.config.js, breaking CI for teammates.

Key Takeaways

• Install Jest with npm install --save-dev jest.
• Plain CommonJS projects need no extra configuration.
• ESM, JSX, and TypeScript projects typically need Babel or ts-jest.
• npx jest --init scaffolds a starter configuration file interactively.