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:** ```typescript // 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:** ```typescript // vitest.config.ts import { defineConfig } from "vitest/config"; export default defineConfig({ test: { globals: true, environment: "node", coverage: { provider: "v8", reporter: [...