Asynchronous Script Loading Callback

Question

http://jsfiddle.net/JamesKyle/HQDu6/

I've created a short function based on Mathias Bynens Optimization of the Google Analytics asynchronous script that goes as following:

function async(src) {
  var d = document, t = 'script',
      o = d.createElement(t),
      s = d.getElementsByTagName(t)[0];
  o.src = '//' + src;
  s.parentNode.insertBefore(o, s);
}

This works great and I've already started using it for several different scripts

// Crazy Egg
async('dnn506yrbagrg.cloudfront.net/pages/scripts/XXXXX/XXXXX.js?' + Math.floor(new Date().getTime() / 3600000));

// User Voice
var uvOptions = {};
async('widget.uservoice.com/XXXXX.js');

// Google Analytics
var _gaq = [['_setAccount', 'UA-XXXXX-XX'], ['_setDomainName', 'coachup.com'], ['_trackPageview']];
async('google-analytics.com/ga.js');

// Stripe
async('js.stripe.com/v1');

The problem comes when I encounter a script that needs to be called after it's loaded:

// Snap Engage
async('snapabug.appspot.com/snapabug.js');
SnapABug.init('XXXXX-XXXXX-XXXXX-XXXXX-XXXXX');

So I figured I'd turn this into a callback function that would be used as so:

async('snapabug.appspot.com/snapabug.js', function() {
    SnapABug.init('XXXXX-XXXXX-XXXXX-XXXXX-XXXXX');
});

I did not expect that this would be difficult for me to do but it has turned out that way.

My question is what is the most efficient way to add a callback without overcomplicating the code.

See the jsfiddle: http://jsfiddle.net/JamesKyle/HQDu6/

http://stackoverflow.com/a/3211647/982924 and http://stackoverflow.com/a/4249346/982924 — RASG, Oct 10 '12 at 13:57

score 58 · Accepted Answer · edited May 23 '17 at 12:34

58

Thanks RASG for https://stackoverflow.com/a/3211647/982924

Async function with callback:

function async(u, c) {
  var d = document, t = 'script',
      o = d.createElement(t),
      s = d.getElementsByTagName(t)[0];
  o.src = '//' + u;
  if (c) { o.addEventListener('load', function (e) { c(null, e); }, false); }
  s.parentNode.insertBefore(o, s);
}

Usage:

async('snapabug.appspot.com/snapabug.js', function() {
    SnapABug.init('XXXXX-XXXXX-XXXXX-XXXXX-XXXXX');
});

jsFiddle

edited May 23 '17 at 12:34

Community

1
1

answered Oct 10 '12 at 14:17

James Kyle

4,138
4
28
41

1

The line `o.u = '//' + u;` should read `o.src = '//' + u;` or else it won't load a single byte. Minifying can be tricky at times. – aefxx Apr 16 '14 at 10:55
2

s.addEventListener should be o.addEventListener – baohouse Jan 29 '15 at 21:12
1

I think this needs more logic to deal with IE8 and before, see for example: http://www.aaronpeters.nl/blog/prevent-double-callback-execution-in-IE9 – Dave Cahill Mar 25 '15 at 08:28
I think this solution is more comprehensive and includes a `Promise` implementation too: https://stackoverflow.com/questions/7718935/load-scripts-asynchronously – Kevin Farrugia Aug 09 '17 at 07:39

Ced · Answer 2 · 2019-03-05T08:41:21.383

A more recent snippet:

async function loadAsync(src) {
    const script = document.createElement('script');
    script.src = src;
    return new Promise((resolve, reject) => {
        script.onreadystatechange = function () {
            if (script.readyState === 'loaded' || script.readyState === 'complete') {
                script.onreadystatechange = null;
                resolve(true);
            }
        };
        document.getElementsByTagName('head')[0].appendChild(script);
    });
}

utilisation

  loadAsync(`https://....js`).then(_ => {
    //  ... script loaded here
  })

James Kyle's answer doesn't take IE9 into account. Here is a modified version of the code I found in the link proposed in the comments. Modify the var baseUrl so it can find the script accordingly.

//for requiring a script loaded asynchronously.
function loadAsync(src, callback, relative){
    var baseUrl = "/resources/script/";
    var script = document.createElement('script');
    if(relative === true){
        script.src = baseUrl + src;  
    }else{
        script.src = src; 
    }

    if(callback !== null){
        if (script.readyState) { // IE, incl. IE9
            script.onreadystatechange = function() {
                if (script.readyState === "loaded" || script.readyState === "complete") {
                    script.onreadystatechange = null;
                    callback();
                }
            };
        } else {
            script.onload = function() { // Other browsers
                callback();
            };
        }
    }
    document.getElementsByTagName('head')[0].appendChild(script);
}

utilisation:

loadAsync('https://www.gstatic.com/charts/loader.js' , function(){
    chart.loadCharts();
    });
// OR relative path
loadAsync('fastclick.js', null, true);

Nice work & thanks for the great answer. Just change "doc" in the last line of the function to "document". — Trev14, Mar 24 '17 at 20:26

score 12 · Answer 3 · answered Apr 13 '19 at 21:40

12

The other answers works well, but aren't super readable or require Promises. Here is my two cents:

function loadScript(src, callback) {
    var script = document.createElement('script');
    script.setAttribute('src', src);
    script.addEventListener('load', callback);
    document.head.appendChild(script);
},

answered Apr 13 '19 at 21:40

Fredrik

3,027
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66

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Small improvement: If you use `script.addEventListener('load', callback,{ once: true });` then the listener will be cleared automatically after the load event has fired. – Humppakäräjät Jan 22 '23 at 17:39

Asynchronous Script Loading Callback

My question is what is the most efficient way to add a callback without overcomplicating the code.

3 Answers3

Async function with callback:

Linked

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