JS Bin// source https://jsbin.com/xojajom

jsbin.xojajom.js

const randomWait = (delay) => new Promise((resolve, reject) => {
    setTimeout(() => {
        if (Math.round(Math.random() * 2) % 2) {
            resolve("ok");
        } else {
            reject(new Error("wrong!"));
        }
    }, delay * 1000);
});

randomWait(5)
    .then((result) => {console.log("Once:",result);})
    .catch((error) => {console.log("Once:",error.message);});

Promise
    .all([randomWait(2), randomWait(1), randomWait(3)])    
    .then((result) => {console.log("Promise.all:",result);})
    .catch((error) => {console.log("Promise.all:",error.message);});