20.1 Unit Testing Fundamentals

Overview

Unit testing is the practice of testing individual units of code (functions, methods, classes) in isolation to ensure they work correctly.

Why Unit Test?

┌─────────────────────────────────────────────────────────────────────────┐
│                    Benefits of Unit Testing                             │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                         │
│  ┌──────────────────┐  ┌──────────────────┐  ┌──────────────────┐      │
│  │   Catch Bugs     │  │  Documentation   │  │  Refactoring     │      │
│  │     Early        │  │    by Example    │  │   Confidence     │      │
│  └──────────────────┘  └──────────────────┘  └──────────────────┘      │
│                                                                         │
│  ┌──────────────────┐  ┌──────────────────┐  ┌──────────────────┐      │
│  │    Faster        │  │   Better Code    │  │   Regression     │      │
│  │   Debugging      │  │     Design       │  │   Prevention     │      │
│  └──────────────────┘  └──────────────────┘  └──────────────────┘      │
│                                                                         │
└─────────────────────────────────────────────────────────────────────────┘

AAA Pattern

Phase	Description	Example
Arrange	Set up test data and conditions	Create test objects, initialize state
Act	Execute the code being tested	Call the function with test data
Assert	Verify the expected outcome	Check return value or state changes

Test Doubles

┌─────────────────────────────────────────────────────────────────────────┐
│                         Test Doubles                                    │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                         │
│  Dummy        → Objects passed but never used                           │
│  ────────────────────────────────────────────────────────────────────   │
│                                                                         │
│  Stub         → Provides canned answers to calls                        │
│  ────────────────────────────────────────────────────────────────────   │
│                                                                         │
│  Spy          → Records calls for verification                          │
│  ────────────────────────────────────────────────────────────────────   │
│                                                                         │
│  Mock         → Pre-programmed with expectations                        │
│  ────────────────────────────────────────────────────────────────────   │
│                                                                         │
│  Fake         → Working implementation (simplified)                     │
│                                                                         │
└─────────────────────────────────────────────────────────────────────────┘

Test Structure

describe('Calculator', () => {
  describe('add', () => {
    it('should add two positive numbers', () => {
      // Arrange
      const calc = new Calculator();

      // Act
      const result = calc.add(2, 3);

      // Assert
      expect(result).toBe(5);
    });
  });
});

Writing Testable Code

Principle	Bad	Good
Pure Functions	Depends on global state	Takes all inputs as arguments
Dependency Injection	Creates dependencies internally	Receives dependencies
Single Responsibility	Does many things	Does one thing well
Avoid Side Effects	Modifies external state	Returns new values

Code Coverage

Metric	Description
Line Coverage	% of lines executed
Branch Coverage	% of branches (if/else) taken
Function Coverage	% of functions called
Statement Coverage	% of statements executed

Summary

•Unit tests verify individual units in isolation
•AAA pattern structures tests clearly
•Test doubles replace real dependencies
•Good tests are fast, isolated, repeatable
•100% coverage doesn't mean bug-free

20.1-Unit-Testing-Fundamentals

20.1 Unit Testing Fundamentals

Overview

Why Unit Test?

AAA Pattern

Test Doubles

Test Structure

Writing Testable Code

Code Coverage

Summary