Typescript

TypeScript Guardrails

These are hard rules. Check every piece of TypeScript you write, review, or modify against this list.

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Types

• DO NOT use enum. Use as const objects + inferred type instead.

  // wrong
  enum Status { Active, Inactive }
  
  // correct
  const STATUS = { Active: 'active', Inactive: 'inactive' } as const;
  type Status = typeof STATUS[keyof typeof STATUS];
  

• DO NOT use any. Use unknown with type guards to narrow.

• Prefer type over interface unless you need declaration merging, extends, or implements.

• DO NOT use type assertions (as X) unless no alternative exists. Narrow with control flow instead. If unavoidable, add a comment explaining why.

  • Only exception: as Record<string, unknown> inside type guards, to access properties on a narrowed object. No other as casts — validate with typeof, in, or equality checks. When checking membership in a const object, widen the array type to ReadonlyArray<string> instead of casting the value:

    function isUser(value: unknown): value is User {
      if (typeof value !== 'object' || value === null) return false;
      const obj = value as Record<string, unknown>; // only acceptable assertion
      const validRoles: ReadonlyArray<string> = Object.values(USER_ROLE);
      return typeof obj.name === 'string' && typeof obj.role === 'string' && validRoles.includes(obj.role);
      // ↑ widen to ReadonlyArray<string> — do NOT cast obj.role as SomeType
    }
    

• Use satisfies for type-safe object literals where you want inference + validation.

  const config = { port: 3000, host: 'localhost' } satisfies ServerConfig;
  

• Explicit return types on all exported functions.

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Imports / Exports

• No barrel files (index.ts re-exports) except designated package entry points.

• Explicit import paths — import { bar } from './utils/bar', not from './utils'.

• No default exports except config files (vite.config.ts, eslint.config.ts, etc.). Use named exports everywhere else.

• ESM only — import/export. Never require() or module.exports.

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Strict Mode

• "strict": true in tsconfig — always. Do not loosen individual strict flags.

• "noUncheckedIndexedAccess": true when the project supports it.

• DO NOT use @ts-ignore. Use @ts-expect-error with an explanation comment if suppression is truly needed.

• DO NOT use non-null assertions (!) unless provably safe. If used, add a comment explaining the proof.

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Patterns

• const by default. let only when reassignment is needed. Never var.

• Exhaustive switches — always include a default case with a never check:

  default: {
    const _exhaustive: never = value;
    throw new Error(`Unhandled case: ${_exhaustive}`);
  }
  

• Discriminated unions over type predicates when modeling variants.

• Readonly by default — use readonly properties, ReadonlyArray<T>, Readonly<T>.

• No classes unless required by a framework (e.g., Angular, NestJS). Prefer functions + closures.

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Naming

Thing	Convention	Example
Files	kebab-case	user-service.ts
Types / Interfaces	PascalCase	UserProfile
Functions / variables	camelCase	getUserById
Constants	UPPER_SNAKE_CASE	MAX_RETRY_COUNT
Schema interfaces	<Name>Schema	UserSchema

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Error Handling

• Never swallow errors — no empty catch blocks. Log, rethrow, or handle explicitly.

• Typed errors — use custom error classes or discriminated union result types.

• Avoid throw in library code when possible. Prefer a Result pattern or explicit error returns:

  type Result<T, E = Error> = { ok: true; value: T } | { ok: false; error: E };