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The satisfies Operator

Check Without Widening

value satisfies Type (TypeScript 4.9+) checks that a value matches a type without changing the inferred type. With an annotation (const x: Type = ...) you lose the specific details; with satisfies you keep them and still get the check.

Example

With the annotation, routesA.home is string | RouteObj, so calling .toUpperCase() needs a check. With satisfies, TypeScript remembers that home is a string and product is an object.

type RouteObj = { path: string; auth: boolean };
type Routes = Record<string, string | RouteObj>;

const routesA: Routes = { home: "/", product: { path: "/product/:id", auth: false } };

const routesB = {
  home: "/",
  product: { path: "/product/:id", auth: false },
  orders: { path: "/orders", auth: true },
} satisfies Routes;

console.log(typeof routesA.home);
console.log(routesB.home.toUpperCase());
console.log(routesB.orders.auth);

Output

string
/
true

satisfies Still Catches Mistakes

Example

A wrong value type is reported (TS2322), exactly like an annotation would.

type Routes = Record<string, string | { path: string; auth: boolean }>;
const routes = {
  home: "/",
  cart: { path: "/cart", auth: "yes" },
} satisfies Routes;

Compiler output (tsc)

Line 4: error TS2322: Type 'string' is not assignable to type 'boolean'.