Module 08: Unit Testing

Duration: 2-3 hours | Difficulty: Intermediate

Unit tests are the foundation of your testing pyramid. They're fast, focused, and provide immediate feedback on your code's correctness.

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What You'll Learn

• Setting up Jest/Vitest for unit testing
• Writing effective unit tests
• Mocking dependencies
• Testing async code
• Achieving meaningful coverage

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Module Lessons

Lesson	Topic	Duration
08.1 Setup	Installing and configuring Jest	30 min
08.2 Writing Tests	Your first unit tests	45 min
08.3 Mocking	Isolating code with mocks	45 min
08.4 Async Testing	Testing promises and async/await	30 min
08.5 Coverage	Measuring test coverage	30 min

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Unit Test Characteristics

Speed:        ⚡ Fast (milliseconds)
Count:        📊 Many (hundreds/thousands)
Scope:        🎯 Single function/class
Dependencies: 🔧 Mocked
Cost:         💰 Cheap to write/maintain

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Example Unit Test

describe('calculateDiscount', () => {
  it('should apply 10% discount for orders over $100', () => {
    // Arrange
    const orderTotal = 150;
    const discountRate = 0.1;

    // Act
    const result = calculateDiscount(orderTotal, discountRate);

    // Assert
    expect(result).toBe(15);
  });

  it('should return 0 for orders at $100 or below', () => {
    expect(calculateDiscount(100, 0.1)).toBe(0);
    expect(calculateDiscount(50, 0.1)).toBe(0);
  });
});

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Prerequisites

Before starting:

• ✅ Completed Module 07
• ✅ Node.js project initialized
• ✅ Comfortable with JavaScript functions

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Let's Begin

Ready to write your first unit tests?

→ Start Lesson 08.1: Setup