satisfies (TS 4.9+) checks that a value conforms to a type without widening the value's inferred type. It validates against a constraint while keeping the precise, literal type for later use — the best of annotation and inference.
= <, | >;
: = { : , : };
a..();
b = { : , : } ;
b..();
b..();
With the annotation, b would be typed as the wide Config. With satisfies, b keeps its exact shape { host: string; port: number } and is checked to be a valid Config.
const routes = {
home: "/",
user: "/users/:id",
} satisfies Record<string, string>;
type RouteName = keyof typeof routes; // "home" | "user" — exact keys preserved
as vs annotation: T — checks, but widens to T (lose specifics).as T — forces the type, no real checking (unsafe).satisfies T — checks against T, keeps the narrow inferred type (safe + precise).satisfies lets you validate that an object matches an interface/constraint while still getting exact literal types for autocomplete, narrowing, and keyof.
It's ideal for config objects, route maps, theme palettes, and constant lookups where you want both correctness and precise inference.