promisify typing sandbox

promisify.ts

type Length<A extends any[]> = A extends { length: infer L } ? L : never;
type Prev<N extends number> = [never, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10][N];
type Last<A extends any[]> = A[Prev<Length<A>>];
type LastParam<F extends (...args: any[]) => any> = Last<Parameters<F>>;
type Callback<T> = (error: Error | null, data?: T | null) => void;
type CallbackData<F> = F extends Callback<infer T> ? T : never;

type BH = (...args: any[]) => any;

const promisify = <H extends BH>(handler: H) => {
  return (...args: any[]) => {
    return new Promise<CallbackData<LastParam<H>>>((resolve, reject) => {
      handler(...args, (err, data) => {
        if (err) {
          reject(err);
        } else {
          resolve(data);
        }
      });
    });
  };
};

const testFunc = (
  p1: string,
  p2: number,
  p3: boolean,
  callback: (err: Error | null, data: string | null) => void
) => {
  if (p3) {
    callback(null, p1 + p2);
  } else {
    callback(new Error("Invalid Parameter"), null);
  }
};

testFunc("a", 1, true, (err, data) => console.log(data));

promisify(testFunc)("a", 1, true)
  .then(data => {})
  .catch(error => {});