Asynchronous Script Loading Callback

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);
},

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);

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