typescript-best-practices

name	typescript-best-practices
description	Idiomatic TypeScript patterns for clean, maintainable code. Use when writing or refactoring TypeScript classes, functions, modules, or async logic. (triggers: **/*.ts, **/*.tsx, class, function, module, import, export, async, promise)

TypeScript Best Practices

Priority: P1 (OPERATIONAL)

Implementation Guidelines

• Naming: Use PascalCase for Classes/Types/Interfaces, camelCase for variables/functions, and UPPER_SNAKE for static constants.
• Functions: Use arrow functions for callbacks/logic; function declaration for top-level exports. Always type public API returns.
• Modules: Use Named exports ONLY to enable better refactoring/auto-imports.
• Async: Use async/await with Promise.all() for parallel execution. Implement try-catch for error handling; type catch(e) as unknown and narrow before use. Avoid .then().catch() chains.
• Classes: Explicitly use private, protected, and public modifiers. Favor composition over inheritance and dependency injection with constructor injection and interfaces over singletons for testability.
• Type Safety: Use never for exhaustiveness checks in switch-cases.
• Optional: Use optional chaining (?.) and nullish coalescing (??) over manual checks.
• Imports: Enforce external packages → internal modules → relative imports order automatically via eslint-plugin-import. Use import type for interfaces/types to ensure better tree-shaking and zero runtime overhead.
• Validation: Use Zod or Tsoa for runtime boundary validation.

Anti-Patterns

• No Default Exports: Use named exports.
• No Implicit Returns: Specify return types.
• No Unused Variables: Enable noUnusedLocals.
• No require: Use ES6 import.
• No Empty Interfaces: Use type or non-empty interface.
• No any: Use unknown or a specific type.
• No Unsafe Mocks: Cast with jest.Mocked<T> or as unknown as T.
• No eslint-disable: Fix root cause; never suppress warnings.

References

See references/examples.md for Immutable Interfaces, Exhaustiveness Checking, Assertion Functions, DI Patterns, and Import Organization.