how to promisify a callback code example

Example: js promisify in browser

function promisify(func, callbackPos) {
  return (...args) => {
    return new Promise((resolve) => {
      const cb = (...args) => {
        resolve(args);
      };
      args.splice(callbackPos ? callbackPos : args.length, 0, cb);
      func(...args);
    });
  };
};