动态加载外部js和css

基本原理：
function loadjscssfile(filename, filetype){
if (filetype=="js"){ //if filename is a external JavaScript file
  var fileref=document.createElement('script')
  fileref.setAttribute("type","text/javascript")
  fileref.setAttribute("src", filename)
}
else if (filetype=="css"){ //if filename is an external CSS file
  var fileref=document.createElement("link")
  fileref.setAttribute("rel", "stylesheet")
  fileref.setAttribute("type", "text/css")
  fileref.setAttribute("href", filename)
}
if (typeof fileref!="undefined")
  document.getElementsByTagName("head")[0].appendChild(fileref)
}

loadjscssfile("myscript.js", "js") //dynamically load and add this .js file
loadjscssfile("javascript.php", "js") //dynamically load "javascript.php" as a JavaScript file
loadjscssfile("mystyle.css", "css") ////dynamically load and add this .css file

注意点：
默认挂在dom的第一个节点下，要保证不影响原有的页面的逻辑。
防止重复加载。
var filesadded="" //list of files already added

function checkloadjscssfile(filename, filetype){
if (filesadded.indexOf("["+filename+"]")==-1){
  loadjscssfile(filename, filetype)
  filesadded+="["+filename+"]" //List of files added in the form "[filename1],[filename2],etc"
}
else
  alert("file already added!")
}

checkloadjscssfile("myscript.js", "js") //success
checkloadjscssfile("myscript.js", "js") //redundant file, so file not added