Indexed Access Types
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type Person = { age: number; name: string; alive: boolean };
typeAge = Person["age"];
type Age = numberThe indexing type is itself a type, so we can use unions, keyof, or other types entirely:
type I1 = Person["age" | "name"];
type I1 = string | number
typeI2 = Person[keyofPerson];
type I2 = string | number | boolean
typeAliveOrName = "alive" | "name";
typeI3 = Person[AliveOrName];
type I3 = string | booleanYou’ll even see an error if you try to index a property that doesn’t exist:
type I1 = Person["alve"];Generated error
Property 'alve' does not exist on type 'Person'.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:
const MyArray = [
{ name:"Alice", age:15 },
{ name:"Bob", age:23 },
{ name:"Eve", age:38 },
];
typePerson = typeofMyArray[number];
type Person = {
name: string;
age: number;
}
typeAge = typeofMyArray[number]["age"];
type Age = number
// Or
typeAge2 = Person["age"];
type Age2 = numberYou can only use types when indexing, meaning you can’t use a const to make a variable reference:
const key = "age";
typeAge = Person[key];Generated error
Type 'key' cannot be used as an index type.'key' refers to a value, but is being used as a type here. Did you mean 'typeof key'?However, you can use a type alias for a similar style of refactor:
type key = "age";
typeAge = Person[key];Last updated on