function loadScript(src, async) {
    return new Promise(function (resolve, reject) {
        var s;
        s = document.createElement('script');
        s.src = src;
        s.async = async;
        s.onload = resolve;
        s.onerror = reject;
        document.head.appendChild(s);
    });
}


loadScript('source', false).then(success => {
   //Run your code here which depends on that file
})