Typescript function signature that converts an array to tuple of mapped type

Question

In typescript, an array can be converted to tuple by

type Arr = any[];
const f = < T extends Arr > (...args: [...T]): [...T] => {
  return args;
}

const a = f(1, 'a'); // a is type of [number, string].

We can also map type by

type TypeMap = {
    'n': number;
    's': string
};

const g = <T extends keyof TypeMap>(args: T): TypeMap[T] => {
    throw null;
}

const b = g('s'); //b is type of string

How can I combine above two requirements into one? I tried

const h = <T extends keyof TypeMap>(...args: [...T[]]): [...TypeMap[T][]] => {
    throw null;
}
const c = h('s', 'n');

However, c is type of (string|number)[] instead of [string, number]. I tried

const h = <T extends (keyof TypeMap)[]>(...args: [...T]): [...TypeMap[T[number]][]] => {
    throw null;
}

but got the same c.

I found a solution using object instead of tuple, but tuple solution are welcome.

const f1 = <T extends keyof TypeMap>(...args: [...T[]]): {[P in T]: TypeMap[P]} => {
    throw null;
}
const {s, n} = f1('s', 'n');

typescript playground

jcalz · Accepted Answer · 2021-01-15 04:21:38Z

1

You want to use a mapped tuple. Given T, a tuple type, {[I in keyof T]: ...T[I]...} will be another tuple type; the I only iterates over numeric indices of T. Note that due to a bug/limitation in TypeScript (see microsoft/TypeScript#27995) the compiler doesn't realize that T[I] will be the element type of T, so you have to use some trick like the Extract utility type.

In your case, from the input tuple T extending Array<keyof TypeMap>, you want the output tuple to have an element of type TypeMap[T[I]] at numeric index I:

const h = <T extends (keyof TypeMap)[]>(...args: [...T]): {
    [I in keyof T]: TypeMap[Extract<T[I], keyof TypeMap>]
} => {
    throw null;
}

const c = h('s', 'n'); // [string, number]

Playground link to code

answered Jan 15, 2021 at 4:21

jcalz

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