管道值：Ajax + JQuery

I have a function in which I execute an ajax request and wait till I get a response and return a value but the value returned is undefined. What is wrong?

            function GetVMData(url_s){ 
                return $.ajax({ 
                       url: url_s, 
                       crossDomain: true,  
                       dataType: 'jsonp', 
                       error: function(xhr, status, error) {  
                           alert('failed') 
                        }  
                   }).pipe(function(data) { return data[4]; }); 
            } 

If I print the value of data[4] within the ajax callback it prints the right value, therefore i know the request is going through but when I try this:

            var cord;
            cord = GetVMData(url).done(function(cpu_USG) { 
            return cpu_USG;
            }); 
            alert(cord)

the value of cord is wrong.

    var cord;
    cord = GetVMData(url).done(function(cpu_USG) { 
    return cpu_USG;
    }); 
    alert(cord)

This code runs asynchronously. So you need to perform everything in the callback, like:

    GetVMData(url).done(function(cpu_USG) { 
        alert(cpu_USG);
    }); 

Here:

var cord;

cord = GetVMData(url).done(function(cpu_USG) { 
  return cpu_USG;
});

alert(cord);

cord contains object, not the value. And by the way, you don't know where ajax calling will be finished, so you should be familiar with idea of callbacks..

As an example:

function makeRequest(url, callback) { 
  $.ajax({
    url: url, 
    crossDomain: true,  
    dataType: 'jsonp',

    error: function(xhr, status, error) {  
      alert('failed') 
    },

    success: callback
  }); 
}

var do_something = function (data) {
  alert(data[4]);
};

cord = makeRequest(url, do_something);