promisse-usage.js

const foo = new Promise((resolve,reject) => {
    const failed = Math.random() > 0.5;
    setTimeout(data => {
        if (failed) {
            reject(data);
        } else {
            resolve(data);
        }
    })
});

foo.then(x => console.log("resolved"))
   .catch(err => console.error("rejected"));