typescript-strict

TypeScript Strict Standard

Rules extracted from 10+ production TypeScript codebases. All projects use strict: true.

CRITICAL: Type Safety Rules

TS-01: Never use any

// BAD
function process(data: any) { return data.value; }
const handler = (e: any) => console.log(e);

// GOOD
function process(data: Record<string, unknown>) { return "value" in data ? data.value : undefined; }
const handler = (e: MouseEvent) => console.log(e.target);

// ACCEPTABLE: Library integration boundaries (shadcn/ui props, Tauri responses)
// Must add a comment explaining why
// eslint-disable-next-line @typescript-eslint/no-explicit-any -- CodeMirror extension type

Exceptions (document each with a comment):

• Third-party library props that genuinely require any
• Generic catch blocks: catch (e: unknown) then narrow

TS-02: Avoid as type assertions

// BAD
const user = response as User;
const el = document.getElementById("root") as HTMLDivElement;

// GOOD: use type guards
function isUser(data: unknown): data is User {
  return typeof data === "object" && data !== null && "id" in data;
}
if (isUser(response)) { /* response is User here */ }

// GOOD: use generics
const el = document.getElementById("root");
if (el instanceof HTMLDivElement) { /* el is HTMLDivElement here */ }

// ACCEPTABLE (rare): When you KNOW the type and can't prove it
// Must be `as unknown as T` with a SAFETY comment
const validated = session as unknown as AuthenticatedSession; // SAFETY: verified user.id exists above

When as is acceptable:

• Prisma generated types requiring cast at boundaries
• After runtime validation (Zod parse → safe to assert)
• Event target narrowing after guard: e.target as Node after if (e.target)

TS-03: Use unknown over any for untyped data

// BAD
function parse(input: any): Config { ... }

// GOOD
function parse(input: unknown): Config {
  if (typeof input !== "object" || input === null) throw new Error("Invalid input");
  // Narrow from unknown
}