Introduction to babel-jest

babel-jest is a Jest transformer that uses Babel to transpile your JavaScript
code during testing. Jest is a popular testing framework, and combining it with Babel allows you
to use the latest JavaScript features while ensuring your code runs correctly in various
environments.

Installation

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

Configuration

Create a babel.config.js file in your project’s root directory, and configure Babel
to use the required presets/plugins.

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

Common API and Usage Examples

Basic Test Setup

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

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

Using ES6+ Features

  
    // sum.js
    export const sum = (a, b) => a + b;

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

Transforming with Babel Plugins

  
    // babel.config.js
    module.exports = {
      presets: ['@babel/preset-env'],
      plugins: ['@babel/plugin-transform-arrow-functions'],
    };

    // example.js
    const foo = () => 'bar';
    console.log(foo()); // 'bar'
  

Using Async/Await

  
    // asyncFunction.js
    export const fetchData = async () => {
      const data = await fetch('https://jsonplaceholder.typicode.com/todos/1');
      return data.json();
    };

    // fetchData.test.js
    import { fetchData } from './asyncFunction';
    test('fetches data correctly', async () => {
      const data = await fetchData();
      expect(data.id).toBe(1);
    });
  

App Example

Here’s a simple app example demonstrating API integration with babel-jest:

App Structure

  
    my-app/
    ├── src/
    │   ├── components/
    │   │   └── Hello.js
    │   ├── utils/
    │   │   └── math.js
    │   └── App.js
    └── tests/
        ├── App.test.js
        └── math.test.js
  

Component Example

  
    // src/components/Hello.js
    import React from 'react';
    const Hello = () => <h1>Hello, World!</h1>;
    export default Hello;
  

Utility Function Example

  
    // src/utils/math.js
    export const add = (a, b) => a + b;
  

Test Files

  
    // tests/App.test.js
    import React from 'react';
    import { render } from '@testing-library/react';
    import Hello from '../src/components/Hello';
    test('renders Hello component', () => {
      const { getByText } = render(<Hello />);
      expect(getByText('Hello, World!')).toBeInTheDocument();
    });

    // tests/math.test.js
    import { add } from '../src/utils/math';
    test('adds 2 + 3 to equal 5', () => {
      expect(add(2, 3)).toBe(5);
    });
  

With these snippets, you have a comprehensive guide to using babel-jest in your
projects for writing and running tests.

Hash: 8f87d046d48cf6ab344d915c6a384533a6b91846ce0de9569ad6ccbee7e48852