A real world example of how JS Promises work (A+ Specification)

example.js

import { getJSON } from './promise_example';

function init() {
  getJSON('my-url.com').then( data => {
    // data, the first param of resolve(), contains
    // the response data from the XHR request
    console.log(data); 
  }).catch( error => {
    // oh no, something went wrong!
    // error is the first param of reject()
    console.error(error
  });
}

init();
  

promise_example.js

export function getJSON(url) {

    return new Promise( (resolve, reject) => {
        // Parse the URL:
        if(url.includes('?')) {
            if(url[url.length - 1] !== '&') {
                url += '&';
            }
            url += '_=' + Date.now();
        }
        else {
            url += '?_=' + Date.now();
        }

        let xhr = new XMLHttpRequest();
        xhr.open('GET', url);
        xhr.setRequestHeader('Accept', 'application/json, text/tavascript, */*; q=0.01');
        xhr.setRequestHeader('X-Requested-With', 'XMLHttpRequest');
        xhr.setRequestHeader('Pragma', 'no-cache');
        xhr.setRequestHeader('Cache-Control', 'no-cache, no-store');

        xhr.onreadystatechange = () => {
            if(xhr.readyState < 4) {
                return;
            }

            if(xhr.status !== 200) {

                reject(xhr.response);
            }
            if(xhr.readyState === 4) {
                resolve(JSON.parse(xhr.response));
            }

        }
        xhr.send();
    });
}

with_async_await.js

import { getJSON } from './promise_example';

async function mutipleCallExample() {
  // Always wrap your async/await calls in try/catch.
  // it's best practice :)
  try {
    const firstCallResponse = await getJSON('one-url.com');
    const secondCallResponse = await getJSON('two-url.com');
    
    // this will return the resolved promises in its own promise
    return { first: firstCallResponse, second: secondCallResponse };
  } catch( error) {
    // If something goes wrong with any of the calls above, then
    // throw an error
    throw new Error(error); 
  }
}

function init() {
   // Async functions return a promise, so you'll need to use .then()
   // to get the response inside.
   multipleCallExample().then( result => {
     console.log(result);
     // This console.log shows:
     // { first: firstCallResponse, second: secondCallResponse }
   }).catch( error => {
     // Something went wrong in our big call!
     console.error(error);
   });
}

init();