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//Add this at the bottom of your page right before </body>
//Will let the page content load before loading js
//Make sure you don't have critical js necessary for loading the page when using this because it will break it
function downloadJSOnload() {
    var element = document.createElement('script');
    element.setAttribute('src', 'a.js');
    document.body.appendChild(element);
}
if (window.addEventListener) {
    window.addEventListener('load', downloadJSOnload, false);
}
else if (window.attachEvent) {
    window.attachEvent('onload', downloadJSOnload);
}
else {
    window.onload = downloadJSOnload;
}

//download multiple files
//use for loop to avoid polyfilling forEach on older browsers
function downloadAllJSOnload() {
    var scripts = ["static/js/a.js", "static/js/b.js"];
    for (var i = 0, len = scripts.length; i < len; i++) {
        var element = document.createElement("script");
        element.setAttribute('src', scripts[i]);
        document.body.appendChild(element);
    }
}