異步加載javascript

  function loadJs(file) {
            var frm = document.getElementById("frm");
            var doc = frm.contentWindow.document;
            var scriptTag = doc.getElementById('loadScript');
            var head = doc.getElementsByTagName("head")[0];
            if(scriptTag)head.removeChild(scriptTag);
            script=document.createElement('script');
            script.src=file;
            script.type='text/javascript';
            script.id='loadScript';
            head.appendChild(script);
            }
       function loadCss(file) {
            var frm = document.getElementById("frm");
            var doc = frm.contentWindow.document;
            var cssTag=doc.getElementById('loadCss');
            var head=doc.getElementsByTagName('head').item(0);
            if(cssTag)head.removeChild(cssTag);
            css=document.createElement('link');
            css.href=file;
            css.rel='stylesheet';
            css.type='text/css';
            css.id='loadCss';
            head.appendChild(css);
        }