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')
}