From http://ilee.co.uk/executing-javascript-files-asynchronously/

async-js-files.js

(function() {
    "use strict";
    function loadJSAsync() {
        var js = document.createElement("script");
        js.type = "text/javascript";
        js.src = "script.js";
        js.async = true;
        // insert the "js" script tag to the DOM here ...
    } 
    if (window.attachEvent) {
        window.attachEvent("onload", loadJSAsync);
    } else {
        window.addEventListener("load", loadJSAsync, false);
    }
})();