typescript callback/promise style async function

callback.ts

type Callback<T = any> = (err?: unknown, result?: T) => void


export async function returnSomething(): Promise<number>;
export function returnSomething(callback: Callback<number>): void;

export function returnSomething(callback?: Callback<number>): void | Promise<number> {
  return asCallback<number>(asyncCall(), callback)
}

async function asyncCall(): Promise<number> {
  return Promise.resolve(1)
}

function asCallback<T>(promise: Promise<T>, cb?: (err?: unknown, result?: T) => void): Promise<T> | void {
  if (cb === undefined) {
    return promise
  }

  promise.then(
    (result) => {
      cb(null, result)
    },
    (err) => {
      cb(err)
    }
  )
}