What is a type guard and when should you write one?

A type guard connects a runtime check to the type system. Built-in checks such as typeof and instanceof narrow automatically. A user-defined guard packages a reusable check in a function whose return type is a type predicate, so the compiler can narrow the argument after the call.

type User = { id: number; name: string };

function isUser(value: unknown): value is User {
  return (
    typeof value === "object" &&
    value !== null &&
    "id" in value &&
    "name" in value &&
    typeof value.id === "number" &&
    typeof value.name === "string"
  );
}

function greet(value: unknown): string {
  if (isUser(value)) {
    return "Hello, " + value.name;
  }
  return "Hello, guest";
}

Built-in and custom guards

• typeof narrows primitives
• instanceof narrows class instances
• in narrows object variants by property presence
• Array.isArray narrows arrays
• A type predicate such as value is User narrows after a custom call

The predicate is a promise to the compiler. If the runtime check is incomplete, the body of the guard will be treated as a User even when it is not. For data from outside the program, validate every field you rely on, or use a schema library that returns a typed result.