What does [keyof T] mean after the closing brace in type definition? [duplicate]

Question

Looking at the Typescript documentation at this URL https://www.typescriptlang.org/docs/handbook/advanced-types.html

type FunctionPropertyNames<T> = {
  [K in keyof T]: T[K] extends Function ? K : never;
}[keyof T];

Can someone explain to me what this [keyof T] after the closing brace mean? Is there any documentation for this syntax?

Answer 1

That's called a "lookup type".

1. keyof X returns all keys of a type

2. if

    interface a {
       foo: never
       bar: string
       baz: number
    }

then type not_here = a['foo'] will be never

but lookup types also support passing keyof Something in, so

a[keyof a] would be the union of all types of as keys/properties, which is never | string | number. Though never has no meaning in there so TS automatically ignores it, resulting in string | number.

(you can of course do a[keyof b], there's no restriction here)

I find that the best way to figure out complex types like this one is to decompose them in steps like I did here:

interface test {
    a: string
    b: () => void
    c: () => void
}

type T_A<T> = {
  [K in keyof T]: T[K]
};

type T_B<T> = {
  [K in keyof T]: T[K] extends Function ? K : never;
};

type T_C<T> = {
  [K in keyof T]: T[K] extends Function ? K : never;
}[keyof T];

type a = T_A<test>
type b = T_B<test>
type c = T_C<test>

With this you can see the individual steps taken to obtain the desired result, which is "the union of keys which are of type Function".