promisify.js

function normalFunc(x,y,callback) {
  const result = x + y;
  callback(result);
}
async function run() {
  const promisedFn = promisify(normalFunc);
  const result = await promisedFn(4,3).then(data => data*data);
  console.log(result);
}
run()

function promisify(fn) {
  return (...args) => {
    return new Promise((resolve) => {
      function cb(val) {
        resolve(val)
      }
      fn.apply(this, args.concat(cb));
    })
  }
}