Typing partial application in TypeScript

partial-application.ts

//========== Testing types ==========

const expectType = <Type>(_: Type): void => void 0;
type TypeEqual<Target, Value> = (<T>() => T extends Target
  ? 1
  : 2) extends <T>() => T extends Value ? 1 : 2
  ? true
  : false;

//========== applyPartial ==========

function applyPartial<
  Func extends (...args: any[]) => any,
  InitialArgs extends unknown[],
>(func: Func, ...initialArgs: InitialArgs) {
  return (...remainingArgs: RemainingArgs<Func, InitialArgs>)
  : ReturnType<Func> => {
    return func(...initialArgs, ...remainingArgs);
  };
}

//----- Test -----

function add(x: number, y: number): number {
  return x + y;
}
const add3 = applyPartial(add, 3);
expectType<TypeEqual<
  typeof add3,
  // The parameter name is preserved!
  (y: number) => number
>>(true);

//========== Utility type ==========

type RemainingArgs<
  Func extends (...args: any[]) => any,
  InitialArgs extends unknown[],
> =
  Func extends (...args: [...InitialArgs, ...infer TrailingArgs]) => unknown
  ? TrailingArgs
  : never
;

//----- Test -----

expectType<TypeEqual<
  RemainingArgs<typeof add, [number]>,
  [y: number]
>>(true);