feat: translate Keyof Type Operator.md in zh-CN

[EnochGao]

translate Keyof Type Operator.md in zh-CN

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Translation of Keyof Type Operator.md

---

title: The Keyof type operator
layout: docs
permalink: /zh/docs/handbook/2/keyof-types.html

oneline: "Using the keyof operator in a type context"

keyof Type operator

keyof An operator accepts an object type and combines the key values of the object to produce a literal string consisting of strings or numbers.
The following type P is | with type "x" "y" is equivalent:

type Point = { x: number; y: number };
type P = keyof Point;
//   ^?

If the type hasstringornumberindex signature,keyofThe type of index will be returned:

type Arrayish = { [n: number]: unknown };
type A = keyof Arrayish;
//   ^?

type Mapish = { [k: string]: boolean };
type M = keyof Mapish;
//   ^?

It is worth noting that in the above example, M be string | number -- This is because the keys of objects in JavaScript are always cast to strings, so obj[0] and obj["0"]is equivalent.

whilekeyofThis becomes especially useful when combined with mapping types, as we'll learn more about later.

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