JavaScript & TypeScript Development

Unified skill for all JS/TS work: types, backends, testing, modern patterns, build tools, and API design.

Task Router

Domain	Reference
TypeScript types, generics, mapped/conditional types, type guards	references/advanced-types.md
Node.js, Express, Fastify, middleware, auth, DB, caching	references/node-backend.md
WebSocket, Socket.io, BullMQ, job queues, GraphQL schema, DataLoader, microservices, AbortController, RabbitMQ, Kafka	references/advanced-backend.md
Jest, Vitest, Playwright, mocking, integration, E2E, coverage	references/testing.md
Snapshot testing, test fixtures, faker, testcontainers, MSW, coverage thresholds	references/testing-advanced.md
ES6+, async/await, FP, destructuring, modules, generators	references/modern-js.md
Proxy, Reflect, Symbol, WeakMap, WeakSet, private class fields, tagged templates, async generators	references/advanced-js-patterns.md
Build tools, bundlers, package managers, linting	This file - Build & Tooling
REST, GraphQL, OpenAPI	This file - API Design
Debugging, profiling, TS errors	This file - Debugging
Vite config, vite.config.ts, esbuild, Turbopack, tsconfig strict mode, noUncheckedIndexedAccess, ESLint flat config	references/build-tools-modern.md
Dependency injection, DI container, tsyringe, inversify, manual DI, composition root, interface injection	references/dependency-injection.md
React Testing Library, userEvent, MSW, mock service worker, screen queries, renderHook, Next.js component testing	references/testing-library-react.md
Redis, ioredis, caching, CacheService, @Cacheable decorator, cache invalidation, pub/sub, session storage	references/caching-redis.md
OpenAPI, Swagger, swagger-jsdoc, API versioning, pagination, cursor pagination, DataLoader, error responses, RFC 7807	references/api-design-patterns.md

TypeScript Advanced Types

• Generics: <T extends HasLength> for constraints, multiple params <T, U>
• Conditional: T extends string ? true : false, infer for extraction
• Mapped: { [P in keyof T]: T[P] }, key remapping with as, value filtering
• Template literal: \on${Capitalize}``, recursive path types
• Utility types: Partial, Required, Readonly, Pick, Omit, Record, ReturnType, Parameters
• Patterns: Type-safe event emitter, API client, builder pattern, discriminated unions, deep readonly/partial, form validation
• Guards: value is string predicates, asserts functions, composable guards
• Best practices: unknown over any, interface for objects, type for unions, strict mode, const assertions
• See references/advanced-types.md for code examples

Node.js Backend

• Frameworks: Express (middleware-based, helmet/cors/compression) or Fastify (high perf, schema validation, Pino)
• Architecture: Controllers (HTTP) -> Services (logic) -> Repositories (data). DI container for wiring
• Middleware: JWT auth + role-based authorize, Zod validation, Redis-backed rate limiting, structured logging
• Errors: AppError hierarchy (400-409), global handler, asyncHandler wrapper
• Database: PostgreSQL connection pool, parameterized queries, transaction pattern, Mongoose for MongoDB
• Caching: Redis CacheService with get/set/invalidate, @Cacheable(ttl) decorator
• Auth: JWT access (15m) + refresh (7d) tokens, bcrypt hashing, token refresh endpoint
• Ops: Graceful shutdown (SIGTERM/SIGINT), health checks, connection pooling, API response format
• See references/node-backend.md for code examples

Testing

• Frameworks: Jest (ts-jest preset), Vitest (Vite-native, vi.fn/mock/spyOn), Playwright (E2E)
• Unit: Pure functions (happy/edge/error), classes with beforeEach, async with rejects.toThrow()
• Mocking: Module mocks, DI interface mocks, spies, partial mocks, return sequences, timer/date mocking
• Integration: supertest with real app, DB setup/teardown, auth flow tests
• E2E: Playwright test() with page.goto/click/fill, expect(page).toHaveURL()
• Frontend: @testing-library/react (render/screen/fireEvent), renderHook + act, waitFor async
• Patterns: AAA, test factories with faker, 80%+ coverage, describe nesting, behavior over implementation
• See references/testing.md for code examples

Modern JavaScript (ES6+)

• Core: Arrow functions (lexical this), destructuring (object/array/nested/defaults), spread/rest, template literals
• Modules: Named/default exports, dynamic import(), conditional loading, tree-shakeable
• Async: Promise.all/allSettled/race/any, async/await, retry with backoff, timeout wrapper, error tuples
• FP: Array methods (map/filter/reduce/flatMap), currying, partial application, memoization, pipe/compose
• Immutability: Spread for updates, structuredClone for deep copy, Object.freeze, never mutate inputs
• Generators: function* with yield, async generators for pagination, lazy pipelines (map/filter/take)
• Operators: Optional chaining ?., nullish coalescing ??, logical assignment ??=/||=/&&=
• Performance: debounce, throttle, AbortController for cancellable fetch
• See references/modern-js.md for code examples

Build Tools & Package Management

Bundlers

• Vite: Dev server with HMR, Rollup-based production builds. Default choice for new projects
• esbuild: Extremely fast, use for libraries and CLI tools. Limited plugin ecosystem
• Webpack: Mature ecosystem, complex config. Use when Vite/esbuild insufficient
• Turbopack: Next.js integrated, Rust-based. Use only within Next.js

Package Managers

• npm: Default, widest compatibility. package-lock.json
• pnpm: Fastest, strict deps via symlinks. Preferred for monorepos. pnpm-lock.yaml
• yarn: Berry (v4) with PnP or node_modules. yarn.lock
• Always pin exact versions in production (--save-exact). Use engines field

Linting & Formatting

• ESLint: Flat config (eslint.config.js). Extend @typescript-eslint for TS projects
• Prettier: .prettierrc with semi, singleQuote, trailingComma
• lint-staged + husky: Pre-commit hooks for lint/format enforcement

API Design

REST

• Nouns for resources: /users, /users/:id, /users/:id/posts
• HTTP verbs: GET (read), POST (create), PUT (replace), PATCH (update), DELETE
• Status codes: 200, 201, 204, 400, 401, 403, 404, 409, 422, 429, 500
• Pagination: ?page=1&limit=20 with response metadata. Versioning: URL prefix /v1/

GraphQL

• Schema-first (SDL) or code-first (type-graphql). DataLoader for N+1 prevention
• Implement depth/complexity limiting to prevent abuse queries

OpenAPI

• Generate from code with swagger-jsdoc or write spec-first with openapi-generator
• Use for client SDK generation, docs, and contract testing

Debugging

Node.js

• --inspect flag + Chrome DevTools or VS Code debugger
• console.time()/timeEnd() for quick measurement, --prof for CPU, --heap-prof for memory

Common TypeScript Errors

• "not assignable": Missing props, wrong types, missing | undefined
• "property does not exist": Use type guard, discriminated union, or optional chaining
• "cannot find module": Check tsconfig.json paths, moduleResolution, types array

Common Pitfalls

• Forgetting await on async calls (returns Promise, not value)
• == vs === (always strict equality)
• this binding in callbacks (use arrow functions)
• Floating promises without .catch() or try/catch