Testing with Jest

Why Testing Matters

Testing ensures your code works as expected and prevents regressions. Jest is a zero-configuration testing framework with built-in mocking and assertion libraries.

Setting Up Jest

Install Jest:

npm install --save-dev jest

Add to package.json:

"scripts": {
    "test": "jest"
  }

Writing Your First Test

// 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);
  });

Matchers

Jest provides various matchers:

• expect(value).toBe(expected) - Strict equality
• expect(value).toEqual(expected) - Deep equality
• expect(value).toBeTruthy() - Truthy values
• expect(array).toContain(item) - Array containment

Testing Asynchronous Code

Use async/await or Promises:

test('async data', async () => {
    const data = await fetchData();
    expect(data).toBeDefined();
  });

Mocking

Mock functions and modules to isolate tests:

const mockFn = jest.fn();
  mockFn.mockReturnValue('mocked value');
  expect(mockFn()).toBe('mocked value');

Mocking Modules

jest.mock('./api');
  const api = require('./api');
  api.fetchUser.mockResolvedValue({ name: 'John' });

Best Practices

• Test behavior, not implementation
• Use descriptive test names
• Arrange-Act-Assert pattern
• Test edge cases and error conditions
• Keep tests fast and independent

Jest's simplicity and features make it an excellent choice for testing JavaScript applications, ensuring code reliability and maintainability.