Promises

promise.js

'use strict';
const http = require('https');

function docs() {
  console.log('The Promise object is used for deferred and asynchronous computations. A Promise represents an operation that hasn\'t completed yet, but is expected in the future.'); 
  console.log("pending: initial state, not fulfilled or rejected.\n fulfilled: meaning that the operation completed successfully.\n rejected: meaning that the operation failed."); 
  console.log('Great notes from Mozilla: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise');
}

function takesLongTime(url, resolve, reject) {
  const callback = (response) => {
    let str = '';
    response.setEncoding('utf8');
    response.on('data', (chunk) => {
      str += chunk;
    });

    response.on('end', () => {
      resolve(str.trim());
    });
  }

  const req = http.get(url, callback);
  req.end();

  req.on('error', (e) => {
    reject(e);
  });
}

function doStuff() {
  return new Promise((resolve, reject) => {
    takesLongTime('https://www.random.org/integers/?num=1&min=1&max=10&col=1&base=10&format=plain&rnd=new', resolve, reject); 
  });
}

function doBadStuff() {
  return new Promise((resolve, reject) => {
    takesLongTime('https://wont_work_at_all', resolve, reject); 
  });
}

//success
doStuff().then((randomNum) => {
  console.log(`My random number: ${randomNum}`);
}).catch((err) => {
  console.error(`Oh noes!: ${err}.  I will never be called!`);
});

//failure
doBadStuff().then((randomNum) => {
  console.log(`My randomNum: ${randomNum}.  I will never be called!`);
}).catch((err) => {
  console.error(`Oh noes!: ${err}`);
});

//race
const a1 = doStuff();
const a2 = doStuff();
const a3 = doStuff();
Promise.race([a1, a2, a3]).then((randomNum) => {
  console.log(`Race: ${randomNum}`);
});

//all
const b1 = doStuff();
const b2 = doStuff();
const b3 = doStuff();
Promise.all([b1, b2, b3]).then((randomNums) => {
  console.log(`All: ${randomNums.join(', ')}`);
});