TSDocWe can use an indexed access type to look up a specific property on another type:
ts
type Person = { age: number; name: string; alive: boolean };
type Age = Person["age"];
// ^?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
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];
// ^?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
// @errors: 2339
type Person = { age: number; name: string; alive: boolean };
// ---cut---
type I1 = Person["alve"];// @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
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"];
// ^?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
// @errors: 2538 2749
type Person = { age: number; name: string; alive: boolean };
// ---cut---
const key = "age";
type Age = Person[key];// @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
type Person = { age: number; name: string; alive: boolean };
// ---cut---
type key = "age";
type Age = Person[key];type Person = { age: number; name: string; alive: boolean };
// ---cut---
type key = "age";
type Age = Person[key];