Design pattern for managing multiple asynchronous JavaScript operations

Question

I\'m looking for a good design pattern for keeping track of a bunch of different asynchronous JavaScript activities (images loading, multiple AJAX calls, sequenced AJAX call

Answer 1

With promises now standard in ES6 and many good promise libraries extending that for both new features and backwards compatibility, it seems that promises are the way to go here.

The solution starts with taking each async operation and creating a wrapper that returns a promise:

For loading an image:

function loadImage(url) {
    return new Promise(function(resolve, reject) {
        var img = new Image();
        img.onload = function() {
            resolve(img);
        };
        img.onerror = img.onabort = function() {
            reject(url);
        };
        img.src = url;
    });
}

For making an Ajax call (simplified version):

function ajaxGet(url) {
    return new Promise(function(resolve, reject) {
        var req = new XMLHttpRequest();
        req.addEventListener("load", resolve);
        req.addEventListener("error", reject);
        req.addEventListener("abort", reject);
        req.open("GET", url);
        req.send();
    });
}

For waiting a particular amount of time before executing an operation, you can even making a promise version of setTimeout() so it can be chained with other promise operations:

// delay, return a promise
// val is optional
function delay(t, val) {
    return new Promise(function(resolve) {
        setTimeout(function() {
            resolve(val);
        }, t);
    });
}

Now, these can be combined to create the logic the question asked for:

There are three images loading. As soon as one specific image is loaded, I want to display it. Once it's been displayed for a certain amount of time, I want to display the second image. The third one goes in a queue for later display.

// start all three images loading in parallel, get a promise for each one
var imagePromises = [url1, url2, url3].map(function(item) {
    return loadImage(item);
});

// display the three images in sequence with a delay between them
imagePromises.reduce(function(p, item) {
    return p.then(function() {
        // when the next image is ready display it
        return item.then(function(img) {
            displayImage(img);
            return delay(15 * 1000);
        });
    });
}, Promise.resolve());

This use of .reduce() shows a classic design pattern for sequencing a series of operations on an array using promises.

There are three AJAX calls that must happen in consecutive order (the output of one is used as part of the input of the next).

and

When the AJAX calls are done, there's a bunch of JS processing of the results to do and then two more images need to get loaded.

var p = ajaxGet(url1).then(function(results1) {
    // do something with results1
    return ajaxGet(url2);
}).then(function(results2) {
    // do something with results2
    return ajaxGet(url3); 
}).then(function(results3) {
    // process final results3 here
    // now load two images and when they are loaded do some display stuff
    return Promise.all(loadImage(imgx), loadImage(imgy)).then(function(imgs) {
        doSomeDisplayStuff(imgs);
    });
});