ajaxPrefilter and ajaxTransport processing simulating image in JQuery

//////////////////////////////////////////////////////////////////
  // options  Is the option of the request                         //
  // originalOptions  Value as provided to Ajax Method unmodified option, therefore, no ajaxSettings The default value in Settings  //
  // jqXHR  Is the request jqXHR object                       //
  //////////////////////////////////////////////////////////////////
  $.ajaxPrefilter("image", function(options, originalOptions, jqXHR) {
    // Convert types by preprocessor 
    if (options.test) {
      options.type = 'GET'
    }
    // Increase the prefix 
    options.url = "http://img.mukewang.com/" + options.url
  });


  ///////////////////////
  //  Request distributor  transports //
  ///////////////////////
  $.ajaxTransport("image", function(s) {
    if (s.type === "GET" && s.async) {
      var image;
      return {
        send: function(_, callback) {
          image = new Image();
          function done(status) {
            if (image) {
              var statusText = (status == 200) ? "success" : "error",
                tmp = image;
              image = image.onreadystatechange = image.onerror = image.onload = null;
              callback(status, statusText, {
                image: tmp
              });
            }
          }
          image.onreadystatechange = image.onload = function() {
            done(200);
          };
          image.onerror = function() {
            done(404);
          };
          show(s.url)
          image.src = s.url;
        },
        abort: function() {
          if (image) {
            image = image.onreadystatechange = image.onerror = image.onload = null;
          }
        }
      };
    }
  });


  $("#test").click(function(){

     // perform 1 An asynchronous HTTP ( Ajax ). 
    var ajax = $.ajax({
      test   : true, // test 
      url   : '547d5a45000156f406000338-590-330.jpg',
      dataType : 'image',
      type   : 'POST',
      data: {
        foo: ["bar1", "bar2"]
      },
      // This object is used for Settings Ajax The context of the associated callback function 
      context: document.body,
      // A callback function used to modify a request before it is sent jqXHR
      beforeSend: function(xhr) {
        xhr.overrideMimeType("text/plain; charset=x-user-defined");
        show(' Local events beforeSend')
      },
      // The callback function completes the request  ( request success  and  error All subsequent calls )
      complete: function() {
        show(' Local events complete')
      },
      error: function() {
        show(' Local events error This function is called when the request fails ')
      },
      success: function() {
        show(' Local events success')
      }
    })

    ajax.done(function() {
      show('done')
    }).fail(function() {
      show('fail')
    }).always(function() {
      show('always')
    })