// old
const getData = callback => {
setTimeout(() => {
const number = Math.random();
callback(number);
}, 5000);
}
// new
const getData = callback => {
let promiseReject;
const promise = new Promise(reject => promiseReject = reject);
setTimeout(() => {
const number = Math.random();
promiseReject(number);
callback && callback(number);
}, 5000);
return promise
}