TypeScript/JavaScript Guide

Applies to: TypeScript 5+, Node.js 20+, ES2022+, React, Server-Side JS

Core Principles

1. Strict TypeScript: Enable all strict flags; treat type errors as build failures
2. Immutability by Default: Use const, readonly, as const, and spread operators; mutate only when profiling demands it
3. Explicit Types at Boundaries: All function signatures, API responses, and public interfaces must have explicit type annotations; infer internally
4. Functional Patterns: Prefer pure functions, map/filter/reduce, and composition over classes and mutation
5. Zero any: Use unknown for truly unknown data, Zod/io-ts for runtime narrowing; every any requires a code-review comment explaining why

Guardrails

TypeScript Configuration

• Enable "strict": true (this activates strictNullChecks, noImplicitAny, strictFunctionTypes, etc.)
• Enable "noUncheckedIndexedAccess": true (arrays and records return T | undefined)
• Enable "noImplicitReturns": true and "noFallthroughCasesInSwitch": true