jquery如何判断文件是否存在

function fileExists(url) {

     var isExists;

     $.ajax({

         url: url,

         async: false,

         type: 'HEAD',

         error: function () {

             isExists = 0;

         },

         success: function () {

             isExists = 1;

         }

     });

     if (isExists == 1) {

         return true;

     } else {

         return false;

     }

}

var Exists=Exists('/cache/view_num/' + uid + '.txt');

if(Exists){

    alert('存在')

}