javascript-testing-patterns

JavaScript Testing Patterns

Comprehensive testing strategies for JavaScript/TypeScript applications.

Framework Setup

Jest Configuration

// 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'],
  coverageThreshold: {
    global: { branches: 80, functions: 80, lines: 80, statements: 80 },
  },
  setupFilesAfterEnv: ['<rootDir>/src/test/setup.ts'],
};

export default config;

Vitest Configuration

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

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

Unit Testing Patterns

Testing Pure Functions

import { describe, it, expect } from 'vitest';

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

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

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

Testing Classes

describe('UserService', () => {
  let service: UserService;

  beforeEach(() => {
    service = new UserService();
  });

  describe('create', () => {
    it('should create a new user', () => {
      const user = { id: '1', name: 'John', email: '[email protected]' };
      const created = service.create(user);

      expect(created).toEqual(user);
      expect(service.findById('1')).toEqual(user);
    });

    it('should throw error if user already exists', () => {
      const user = { id: '1', name: 'John', email: '[email protected]' };
      service.create(user);
      expect(() => service.create(user)).toThrow('User already exists');
    });
  });
});