Lessons · TypeScript · keyof: the names of the keys

The keys as a type

keyof T is the union of T's property names as string literals: keyof { x: number; y: number } is 'x' | 'y'.

Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.

What it is for

A getter that takes a property name should only accept real property names. keyof makes the compiler check the name and know the result's type.

Pair keyof with a generic: <T, K extends keyof T>(obj: T, key: K) returns T[K], the exact type of that property.

type Point = { x: number; y: number };

Two keys.

type Key = keyof Point;

'x' | 'y'.

function getKey(p: Point, k: Key): number { return p[k]; }

Only real keys are accepted.

getKey({ x: 1, y: 2 }, "x");

Fine.

getKey({ x: 1, y: 2 }, "z");

Error: 'z' is not a key of Point.

A typed property picker.

function pickKey<T, K extends  T>(obj: T, key: K) { return obj[key]; }

keyof a type with an index signature is string | number, not your field names. It lists only the keys the type declares.

Practise keyof: the names of the keys on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.