Generalizing Solutions with Test Driven Development

Welcome back to our course on Test-Driven Development (TDD) in TypeScript using Jest. In our previous lesson, we introduced the fundamentals of TDD and the Red-Green-Refactor workflow. Now, we will advance our TDD skills by focusing on generalizing solutions and enhancing the complexity of our testing scenarios.

As a brief reminder, TDD involves a repetitive cycle known as Red-Green-Refactor:

• Red: Write a failing test to clarify the new functionality you aim to implement.
• Green: Develop the smallest amount of code needed to make that test pass.
• Refactor: Clean up the code, enhancing its quality while maintaining its functionality and ensuring all tests remain passing.

In this lesson, we will expand upon the sum function, demonstrating how to generalize it while following these TDD principles.

Before we dive into coding, let's review our current setup. You are already familiar with the sum function from src/math.ts and its corresponding test in test/math.test.ts:

app/test/math.test.ts

import { sum } from '../src/math';

describe('sum function', () => {
  it('should add two numbers correctly', () => {
    // Act
    const result = sum(2, 3);
    
    // Assert
    expect(result).toBe(5);
  });
});

app/src/math.ts

export function sum(a: number, b: number): number {
  return 5;
}

This existing setup serves as a foundation. Now, we'll focus on expanding your understanding by generalizing the approach using TDD principles. Understanding where you've come from will help ensure future changes enhance our function without straying too far from the core logic.

To embrace the Red phase, let's introduce a new test case that will fail.

Update math.test.ts to include more input scenarios:

app/test/math.test.ts

import { sum } from '../src/math';

describe('sum function', () => {
  it('should add two numbers correctly', () => {
    // Act
    const result = sum(2, 3);
    
    // Assert
    expect(result).toBe(5);
  });
  
  it('should add two more numbers correctly', () => {
    // Act
    const result = sum(5, 6);
    
    // Assert
    expect(result).toBe(11);
  });
});

By including a new scenario with new inputs, this step is intentionally set to fail to define our target goal clearly.

Running this test:

Output:

FAIL test/math.test.ts
  sum function
    ✓ should add two numbers correctly (3ms)
    ✕ should add two more numbers correctly (4ms)

  ● sum function › should add two more numbers correctly

    expect(received).toBe(expected) // Object.is equality

This failure confirms that the new functionality needs addressing.