javascript-testing-patterns

JavaScript Testing Patterns

Comprehensive guide for implementing robust testing strategies in JavaScript/TypeScript applications using modern testing frameworks and best practices.

When to Use This Skill

• Setting up test infrastructure for new projects
• Writing unit tests for functions and classes
• Creating integration tests for APIs and services
• Implementing end-to-end tests for user flows
• Mocking external dependencies and APIs
• Testing React, Vue, or other frontend components
• Implementing test-driven development (TDD)
• Setting up continuous testing in CI/CD pipelines

Testing Frameworks

Jest - Full-Featured Testing Framework

Setup:

// jest.config.ts
import type { Config } from 'jest';

const config: Config = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  roots: ['<rootDir>/src'],
  testMatch: ['**/__tests__/**/*.ts', '**/?(*.)+(spec|test).ts'],
  collectCoverageFrom: [
    'src/**/*.ts',
    '!src/**/*.d.ts',
    '!src/**/*.interface.ts',
  ],
  coverageThreshold: {
    global: {
      branches: 80,
      functions: 80,
      lines: 80,
      statements: 80,
    },
  },
  setupFilesAfterEnv: ['<rootDir>/src/test/setup.ts'],
};

export default config;

Vitest - Fast, Vite-Native Testing

Setup:

// vitest.config.ts
import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    globals: true,
    environment: 'node',
    coverage: {
      provider: 'v8',
      reporter: ['text', 'json', 'html'],
      exclude: ['**/*.d.ts', '**/*.config.ts', '**/dist/**'],
    },
    setupFiles: ['./src/test/setup.ts'],
  },
});

Unit Testing Patterns

Pattern 1: Testing Pure Functions

// utils/calculator.ts
export function add(a: number, b: number): number {
  return a + b;
}

export function divide(a: number, b: number): number {
  if (b === 0) {
    throw new Error('Division by zero');
  }
  return a / b;
}

// utils/calculator.test.ts
import { describe, it, expect } from 'vitest';
import { add, divide } from './calculator';

describe('Calculator', () => {
  describe('add', () => {
    it('should add two positive numbers', () => {
      expect(add(2, 3)).toBe(5);
    });

    it('should add negative numbers', () => {
      expect(add(-2, -3)).toBe(-5);
    });

    it('should handle zero', () => {
      expect(add(0, 5)).toBe(5);
      expect(add(5, 0)).toBe(5);
    });
  });

  describe('divide', () => {
    it('should divide two numbers', () => {
      expect(divide(10, 2)).toBe(5);
    });

    it('should handle decimal results', () => {
      expect(divide(5, 2)).toBe(2.5);
    });

    it('should throw error when dividing by zero', () => {
      expect(() => divide(10, 0)).toThrow('Division by zero');
    });
  });
});