Operators

satisfies

Checks a value against a type without widening it to that type.

What it is

Added in 4.9 to solve a real dilemma: annotating a constant gets it checked but throws away everything specific about it, while leaving it unannotated keeps the detail and checks nothing. `satisfies` does both — the constraint is enforced and the inferred type is the narrow one you actually wrote.

Examples

```
typeof routes
```
→{ readonly home: "/"; readonly about: "/about"; }
```
typeof widened
```
→{ [x: string]: string; }
The annotation collapses to a bare index signature — the constraint was checked and every literal thrown away doing it. That loss is what `satisfies` exists to prevent.
```
keyof typeof routes
```
→"home" | "about"

Each resolved type above was printed by TypeScript 5.9.3, not written by hand.

What it does not do

It does not change the value or the emitted JavaScript. Like every type-level construct it is erased, and it never runs.
It is not a cast. `as` tells the compiler to stop arguing; `satisfies` asks it to check and then keeps quiet about the result.

Takeaway

Reach for `as const satisfies X` on configuration objects. You get the constraint enforced and the literal types kept, which used to be a choice.