What TypeScript practices do you follow in production?

Good TypeScript is less about clever type gymnastics and more about making invalid states hard to represent. The compiler should enforce the important contracts, while the code stays readable for the next person. Types are documentation that cannot drift silently.

• Enable strict mode and fix errors with narrowing, not assertions
• Annotate public APIs, exported functions and shared data shapes
• Let local variables and simple callbacks be inferred
• Use unknown plus validation for external data instead of any
• Model finite states with discriminated unions and check them exhaustively
• Keep generics only where they preserve a real relationship
• Avoid deep conditional types unless they remove meaningful duplication

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

function parseUser(input: unknown): Result<{ id: number }> {
  if (
    typeof input === "object" &&
    input !== null &&
    "id" in input &&
    typeof input.id === "number"
  ) {
    return { ok: true, value: { id: input.id } };
  }

  return { ok: false, error: "Invalid user" };
}

This example combines several practices: unknown at the boundary, a runtime check, a discriminated result and a precise success type. Callers must handle both branches, and the success branch carries the validated value. The type is doing real work rather than decorating the code.