Jest for JavaScript

Jest is a fast, flexible, and popular testing framework for JavaScript projects. This guide will help you set up Jest in an existing JavaScript project to start writing and running tests effortlessly.

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Prerequisites

Ensure the following dependencies are installed in your project:

1. Jest: Install Jest in your project and ts-jest, a TypeScript preprocessor for Jest:

   npm install jest --save-dev

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2. Babel (Optional): If your project uses modern JavaScript syntax (e.g., ES6+), install Babel and the Jest Babel preset:

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

Then, create or update your .babelrc file with the following configuration:

{
  "presets": ["@babel/preset-env"]
}

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Configuration Steps

1. Add Jest Configuration

Create a jest.config.js file in the root of your project with the following content:

module.exports = {
  testEnvironment: 'node',
  testMatch: ['**/?(*.)+(spec|test).js?(x)'], // Matches test files
  collectCoverageFrom: [
    'src/**/*.{js,jsx}', // Include source files
    '!src/**/*.d.ts',    // Exclude declaration files
  ],
  coveragePathIgnorePatterns: [
    '/node_modules/'     // Exclude node modules
  ],
};

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2. Add Test Scripts

Update your package.json to include test scripts:

"scripts": {
  "test": "jest",
  "test:watch": "jest --watch",
}

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3. Write a Test

Create a test file in your project, for example, src/sample.test.js:

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

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

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4. Run Tests

Run your tests using the following script:

npm test

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With this setup, your JacaScript project is ready to leverage Jest for robust and efficient unit testing. Happy testing!