Level operations for tuple labels

Is it possible in TypeScript 4 to achieve something similar to the code snippet below using labelled tuples?

type stringProperties<T extends {}> = {[k in keyof T]: string}

This concept would allow me to transform a type like

[foo: boolean, bar: number, baz: any]
into
[foo: string, bar: string, baz:string]

Currently, I am struggling to find a way to dynamically capture the labels (as they are not directly present in keyof) and unsure about how to append another label:type pair to an existing tuple type.

I am familiar with the method shown below to add to an unlabeled tuple, however, it always sets the label as first.

export type Prepend<E, T extends any[]> =
    ((first: E, ...args: T) => any) extends ((...args: infer U) => any)
    ? U
    : never

Answer №1

Utilize mapped tuple types to easily change element types while preserving their labels:

type T1 = stringProperties<[foo: boolean, bar: number, baz: any]> 
// [foo: string, bar: string, baz: string]

Although you cannot directly extract function parameter names, adding labeled elements is still feasible with TS 4.0 variadic tuple types:

type Prepend<E extends [unknown], A extends any[]> = [...E, ...A]
type Append<E extends [unknown], A extends any[]> = [...A, ...E]
// ... customize as needed

type T2 = Prepend<[customLabel: string], A> 
// [customLabel: string, foo: boolean, bar: number, baz: any]
type T3 = Append<[customLabel: string], A> 
// [foo: boolean, bar: number, baz: any, customLabel: string]
type T4 = Prepend<[customLabel: string], stringProperties<A>> // mix and match
// [customLabel: string, foo: string, bar: string, baz: string]
... Then proceed to use these types as function parameters:
function foo(...args: T4) {}
// function foo(customLabel: string, foo: string, bar: string, baz: string): void

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