Testing Best Practices

Guides writing effective tests for TypeScript projects.

Test Structure

Use the Arrange-Act-Assert pattern:

test('adds two numbers', () => {
  // Arrange
  const a = 2, b = 3;

  // Act
  const result = add(a, b);

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

What to Test

• Pure functions: all input/output combinations
• API handlers: success, error, edge cases
• Components: rendering, user interactions, state changes
• Integration: database queries, external API calls (mocked)

What Not to Test

• Implementation details (private methods, internal state)
• Third-party library behavior
• Trivial getters/setters

Running Tests

bun test                    # Run all tests
bun test --watch            # Watch mode
bun test path/to/file.ts   # Single file
bun test --coverage         # With coverage report

Mocking

Use mock() for external dependencies:

import { mock } from 'bun:test';
const fetchMock = mock(() => Promise.resolve({ ok: true }));