动态加载外部css或js文件

      By referencing the HEAD element of the page first and then calling appendChild(), this means the newly created element is added to the very end of the HEAD tag. Furthermore, you should be aware that no existing element is harmed in the adding of the new element- that is to say, if you call loadjscssfile("myscript.js", "js") twice, you now end up with two new "script" elements both pointing to the same Javascript file. This is problematic only from an efficiency standpoint, as you'll be adding redundant elements to the page and using unnecessary browser memory in the process. A simple way to prevent the same file from being added more than once is to keep track of the files added by loadjscssfile(), and only load a file if it's new:

      Here I'm just crudely detecting to see if a file that's set to be added already exists within a list of added files' names stored in variable filesadded before deciding whether to proceed or not.

      Ok, moving on, sometimes the situation may require that you actually remove or replace an added .js or .css file. Lets see how that's done next.