Ladies and gentlemen, my web page some resources, need to determine whether there is any, needs to use ajax to the server,
For example,
Img, had hoped at www.baid.com/abc.img
Or mp3, had hoped at www.baid.com/abc.mp3
But it may not be so in js judgment first, have the url resources, if there is a display or play, if not,
Ajax {" "" "" "" "} send a json to the server, the server dynamically generated,

Could you tell me how to judge the existence of resources and,
thank you