Approach for organizing various asynchronous JavaScript tasks

Question

Approach for organizing various asynchronous JavaScript tasks

What is a recommended design pattern for managing various asynchronous JavaScript tasks such as loading images, multiple AJAX calls, and sequenced AJAX calls? How can I handle these activities efficiently without relying heavily on custom callbacks and state variables?

Problem Scenario

I am facing challenges with a startup sequence that involves several asynchronous processes like image loading, timer waits, AJAX calls, and initialization tasks. Some tasks can run simultaneously while others need to be executed in a specific order. Currently, I am using callback functions and global state variables to track the completion status, but this approach leads to complexity and introduces bugs due to sequencing issues.

Debugging becomes difficult as setting breakpoints alters the execution flow significantly. It's like dealing with the Heisenberg uncertainty principle where observing changes the behavior of the system, making it hard to pinpoint errors.

To illustrate the problem further:

Three images are loading, and I want to display them sequentially after specific conditions are met.
There are three consecutive AJAX calls where each response influences the next call.
After processing the AJAX results, two more images need to be loaded.
Additional display operations follow once those images are ready.
Based on the duration one image has been displayed, a decision is made to show the next image or wait longer.

Each step includes success and error handlers, some triggering alternative code paths when needed.

I recently came across YUI's AsyncQueue, which helps with ordering async operations, but it doesn't fully address the type of decision-making complexities I encounter in my scenario.

javascript design-patterns asynchronous callback

Answer 1

Answer №1

Check out the idea of Promises/A, which is implemented in jQuery using the jQuery.Deferred object.

There's a helpful article that demonstrates how this concept can be beneficial in your scenario. I actually posted a similar question some time ago.

Answer 2

Check out the idea of Promises/A, which is implemented in jQuery using the jQuery.Deferred object.

There's a helpful article that demonstrates how this concept can be beneficial in your scenario. I actually posted a similar question some time ago.

Answer 3

Answer №2

Embracing the widespread adoption of promises in ES6 and various promise libraries that enhance functionality, it is evident that promises are the preferred approach.

The strategy commences by encapsulating each asynchronous operation within a wrapper that yields a promise:

To load 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 (abridged 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();
    });
}

To introduce a delay before executing an action, a promise-based version of setTimeout() can facilitate chaining with other promises:

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

These components can then be orchestrated to address the logic outlined in the query:

Three images are loading concurrently. Upon the completion of loading a specific image, it should be displayed. Subsequently, after displaying for a defined duration, the second image should be shown, while the third image waits for later display.

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

// sequentially display the three images with intervals
imagePromises.reduce(function(p, item) {
    return p.then(function() {
        // upon readiness of next image, display it
        return item.then(function(img) {
            displayImage(img);
            return delay(15 * 1000);
        });
    });
}, Promise.resolve());

The usage of .reduce() exemplifies a classic design pattern for sequencing operations on an array utilizing promises.

Three AJAX calls must execute consecutively, where output from one forms part of input for the subsequent request.

Additionally,

Upon completion of the AJAX requests, extensive processing of results is required followed by loading two additional images.

var p = ajaxGet(url1).then(function(results1) {
    // process results1
    return ajaxGet(url2);
}).then(function(results2) {
    // process results2
    return ajaxGet(url3); 
}).then(function(results3) {
    // perform final processing on results3
    // proceed to load two more images and handle their display post-loading
    return Promise.all(loadImage(imgx), loadImage(imgy)).then(function(imgs) {
        doSomeDisplayStuff(imgs);
    });
});

Answer 4

Embracing the widespread adoption of promises in ES6 and various promise libraries that enhance functionality, it is evident that promises are the preferred approach.

The strategy commences by encapsulating each asynchronous operation within a wrapper that yields a promise:

To load 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 (abridged 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();
    });
}

To introduce a delay before executing an action, a promise-based version of setTimeout() can facilitate chaining with other promises:

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

These components can then be orchestrated to address the logic outlined in the query:

Three images are loading concurrently. Upon the completion of loading a specific image, it should be displayed. Subsequently, after displaying for a defined duration, the second image should be shown, while the third image waits for later display.

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

// sequentially display the three images with intervals
imagePromises.reduce(function(p, item) {
    return p.then(function() {
        // upon readiness of next image, display it
        return item.then(function(img) {
            displayImage(img);
            return delay(15 * 1000);
        });
    });
}, Promise.resolve());

The usage of .reduce() exemplifies a classic design pattern for sequencing operations on an array utilizing promises.

Three AJAX calls must execute consecutively, where output from one forms part of input for the subsequent request.

Additionally,

Upon completion of the AJAX requests, extensive processing of results is required followed by loading two additional images.

var p = ajaxGet(url1).then(function(results1) {
    // process results1
    return ajaxGet(url2);
}).then(function(results2) {
    // process results2
    return ajaxGet(url3); 
}).then(function(results3) {
    // perform final processing on results3
    // proceed to load two more images and handle their display post-loading
    return Promise.all(loadImage(imgx), loadImage(imgy)).then(function(imgs) {
        doSomeDisplayStuff(imgs);
    });
});

Answer 5

Answer №3

While Promises help streamline code, leveraging Generators offers even greater possibilities. I recently published a detailed guide on integrating Generators in your JavaScript workflow titled Mastering Async Workflows with ES6 Generators. By following the techniques outlined in the post, you can simplify handling complex asynchronous operations and prepare yourself for the future capabilities of await expected to arrive in ES7.

Answer 6

While Promises help streamline code, leveraging Generators offers even greater possibilities. I recently published a detailed guide on integrating Generators in your JavaScript workflow titled Mastering Async Workflows with ES6 Generators. By following the techniques outlined in the post, you can simplify handling complex asynchronous operations and prepare yourself for the future capabilities of await expected to arrive in ES7.

Approach for organizing various asynchronous JavaScript tasks

Problem Scenario

Answer №1

Answer №2

Answer №3

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