Setting Up a Testing Environment

Before diving into TDD, you need a testing framework for JavaScript. Here are some popular choices:

• Jest: A widely used and highly customizable framework provided by Facebook. It's known for its simplicity, rich features, and excellent documentation.
• Mocha: A flexible and extensible framework that allows for more granular control over test organization and execution.
• Jasmine: A behavior-driven testing framework that focuses on describing the expected behavior of your code in a more human-readable format.

Example: Setting Up Jest

We'll use Jest for this example. Follow these steps:

1. Install Jest:

   npm install --save-dev jest

2. Create a `jest.config.js` file:

   
           module.exports = {
             preset: 'ts-jest',
             testEnvironment: 'node',
             roots: ['/src'],
             testMatch: ['**/__tests__/**/*.+(ts|tsx|js)', '**/?(*.)+(spec|test).+(ts|tsx|js)'],
             transform: {
               '^.+\\.(ts|tsx)$': 'ts-jest'
             }
           };
           

3. Create a test file: (e.g., `sum.test.js`)

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

4. Run tests:

   npm test

Writing Tests: A Practical Example

Let's illustrate TDD with a simple function that reverses a string.

1. Write a Failing Test (Red)

    // reverseString.test.js
    const reverseString = require('./reverseString');

    test('reverses a string', () => {
      expect(reverseString('hello')).toBe('olleh');
    });
    

When you run this test, it will fail because the `reverseString` function doesn't exist yet.

2. Implement the Function (Green)

    // reverseString.js
    function reverseString(str) {
      return str.split('').reverse().join('');
    }

    module.exports = reverseString;
    

Now, the test should pass, turning the indicator to green.

3. Refactor for Clarity (Refactor)

The current implementation is functional, but it could be more readable. Let's refactor it:

    // reverseString.js
    function reverseString(str) {
      let reversed = '';
      for (let i = str.length - 1; i >= 0; i--) {
        reversed += str[i];
      }
      return reversed;
    }

    module.exports = reverseString;
    

Ensure that all tests still pass after the refactoring.

By consistently following this Red-Green-Refactor cycle, you can develop well-tested and reliable JavaScript code.