add original doc

Author: bumkeyy

docs/documentation/ko/handbook-v2/Type Manipulation/Indexed Access Types.md

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+---
+title: Indexed Access Types
+layout: docs
+permalink: /docs/handbook/2/indexed-access-types.html
+oneline: "Using Type['a'] syntax to access a subset of a type."
+---
+
+We can use an _indexed access type_ to look up a specific property on another type:
+
+```ts twoslash
+type Person = { age: number; name: string; alive: boolean };
+type Age = Person["age"];
+//   ^?
+```
+
+The indexing type is itself a type, so we can use unions, `keyof`, or other types entirely:
+
+```ts twoslash
+type Person = { age: number; name: string; alive: boolean };
+// ---cut---
+type I1 = Person["age" | "name"];
+//   ^?
+
+type I2 = Person[keyof Person];
+//   ^?
+
+type AliveOrName = "alive" | "name";
+type I3 = Person[AliveOrName];
+//   ^?
+```
+
+You'll even see an error if you try to index a property that doesn't exist:
+
+```ts twoslash
+// @errors: 2339
+type Person = { age: number; name: string; alive: boolean };
+// ---cut---
+type I1 = Person["alve"];
+```
+
+Another example of indexing with an arbitrary type is using `number` to get the type of an array's elements.
+We can combine this with `typeof` to conveniently capture the element type of an array literal:
+
+```ts twoslash
+const MyArray = [
+  { name: "Alice", age: 15 },
+  { name: "Bob", age: 23 },
+  { name: "Eve", age: 38 },
+];
+
+type Person = typeof MyArray[number];
+//   ^?
+type Age = typeof MyArray[number]["age"];
+//   ^?
+// Or
+type Age2 = Person["age"];
+//   ^?
+```
+
+You can only use types when indexing, meaning you can't use a `const` to make a variable reference:
+
+```ts twoslash
+// @errors: 2538 2749
+type Person = { age: number; name: string; alive: boolean };
+// ---cut---
+const key = "age";
+type Age = Person[key];
+```
+
+However, you can use a type alias for a similar style of refactor:
+
+```ts twoslash
+type Person = { age: number; name: string; alive: boolean };
+// ---cut---
+type key = "age";
+type Age = Person[key];
+```