Operators

keyof

The union of a type’s keys.

What it is

One of the two operators every object transformation is built from — `keyof` gets the keys out, indexed access reads the values back. The result is a union of literal types in no particular order, and there is deliberately no way to ask for "the first key": order is not something an object type guarantees.

Examples

```
keyof User
```
→keyof User
```
keyof { a: 1 }
```
→"a"
```
keyof string[]
```
→keyof string[]
An array has every array method as a key, plus `number`. This is rarely what you wanted — `T[number]` is.

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

What it does not do

It does not give you an ordered list. Object key order is not part of the type, so nothing can depend on it.
It does not skip inherited or built-in members. `keyof` an array or a class instance includes everything the type declares, methods and all.

Takeaway

Reach for `keyof T` when you want the names and `T[keyof T]` when you want the values. Almost every mapped type starts with one of those two.