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TypeScript Tutorial · Generics and Type Manipulation · Lesson 21 of 25

TypeScript Type Assertions and satisfies

as, non-null assertions, as const and the satisfies operator.

Type assertions

value as Type tells the compiler "trust me". It performs no runtime check — if you're wrong, you get a runtime bug. Prefer narrowing; use assertions only when you know more than the compiler (e.g. DOM lookups).

Example 1Try it yourself
const raw: unknown = JSON.parse('{"id": 1, "name": "Ada"}');
const user = raw as { id: number; name: string };
console.log(user.name);

const wrong = JSON.parse('{"id": 1}') as { id: number; name: string };
console.log(wrong.name?.toUpperCase());   // compiles, but name is really undefined

const scores = new Map([["ada", 10]]);
const adaScore = scores.get("ada")!;      // we know the key exists
console.log(adaScore + 1);

Output

Ada
undefined
11

The non-null assertion value! removes null/undefined from a type — the same caveat applies. See the type assertion output puzzle.

as const

as const makes literals as narrow as possible and everything readonly.

satisfies

satisfies checks that a value matches a type without widening the value's inferred type — you keep autocomplete for the exact keys:

Example 2Try it yourself
type Route = { path: string; auth?: boolean };

const routes = {
  home: { path: "/" },
  dashboard: { path: "/dashboard", auth: true },
} satisfies Record<string, Route>;

console.log(routes.dashboard.path);   // autocomplete knows "home" and "dashboard"
const sizes = ["S", "M", "L"] as const;
console.log(sizes.length, sizes[2]);

Output

/dashboard
3 L

Test yourself

Does value as User check the value at runtime?

  1. Yes
  2. No — it only changes the compile-time type
Show answer

Answer: No — it only changes the compile-time type

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