add original doc

Author: bumkeyy

docs/documentation/ko/handbook-v2/Type Manipulation/Keyof Type Operator.md

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+---
+title: Keyof Type Operator
+layout: docs
+permalink: /docs/handbook/2/keyof-types.html
+oneline: "Using the keyof operator in type contexts."
+---
+
+## The `keyof` type operator
+
+The `keyof` operator takes an object type and produces a string or numeric literal union of its keys.
+The following type P is the same type as "x" | "y":
+
+```ts twoslash
+type Point = { x: number; y: number };
+type P = keyof Point;
+//   ^?
+```
+
+If the type has a `string` or `number` index signature, `keyof` will return those types instead:
+
+```ts twoslash
+type Arrayish = { [n: number]: unknown };
+type A = keyof Arrayish;
+//   ^?
+
+type Mapish = { [k: string]: boolean };
+type M = keyof Mapish;
+//   ^?
+```
+
+Note that in this example, `M` is `string | number` -- this is because JavaScript object keys are always coerced to a string, so `obj[0]` is always the same as `obj["0"]`.
+
+`keyof` types become especially useful when combined with mapped types, which we'll learn more about later.