Creating a Promise

by ddrraaggaa

HTML

<!-- Learn about this code on MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise -->

<button id="btn">Make a promise!</button>
<div id="log"></div>

JavaScript

'use strict';
var promiseCount = 0;

function testPromise() {
  var thisPromiseCount = ++promiseCount;

	var log = document.getElementById('log');
	
	log.insertAdjacentHTML('beforeend', thisPromiseCount +
			') Started (<small>Sync code started</small>)<br/>');

	// We make a new promise: we promise a numeric count of this promise, starting from 1 (after waiting 3s)
	var p1 = new Promise(
		// The resolver function is called with the ability to resolve or
		// reject the promise
		function(resolve, reject) {
			log.insertAdjacentHTML('beforeend', thisPromiseCount +
					') Promise started (<small>Async code started</small>)<br/>');
			// This is only an example to create asynchronism
			window.setTimeout(
				function() {
						// We fulfill the promise !
						resolve(thisPromiseCount);
				}, Math.random() * 2000 + 1000);
			}
    );

    // We define what to do when the promise is resolved/fulfilled with the then() call,
    // and the catch() method defines what to do if the promise is rejected.
    p1.then(
        // Log the fulfillment value
        function(val) {
            log.insertAdjacentHTML('beforeend', val +
                ') Promise fulfilled (<small>Async code terminated</small>)<br/>');
        })
    .catch(
        // Log the rejection reason
        function(reason) {
            console.log('Handle rejected promise ('+reason+') here.');
        });

    log.insertAdjacentHTML('beforeend', thisPromiseCount +
        ') Promise made (<small>Sync code terminated</small>)<br/>');
}if ("Promise" in window) {
  var btn = document.getElementById("btn");
  btn.addEventListener("click",testPromise);
} else {
  log = document.getElementById('log');
  log.innerHTML = "Live example not available as your browser doesn't support the <code>Promise<code> interface.";
}