ajax调用何时完全完成？

Question

when any ajax call is done by clients , Clients uploads data to server(HTTP request) and then receive data from server(HTTP Response), when the data is received totally by clients, the ajax request is consider successful and success callback function is executed ,

But when i try to track ,this data transfer between client and server , by using xhr in $.ajax call

xhr: function(){

    var xhr = $.ajaxSettings.xhr() ;

    xhr.upload.onprogress = function(e){console.log('upload progress',e.loaded/e.total*100) } ;
     xhr.onprogress =  function(e){console.log('total downloaded',e.loaded)}
    return xhr ;

In this ajax request , server is sending 277695 bytes file , But The data is not completely downloaded, still it says XHR finished loading , Why after some part is downloaded first , And XHR request is become complerted ,

We can see here server a is sending a file of length 277695 bytes , But only the 7946 bytes downloaded , and it says xhr finished loading , I was expecting , all file (277695 bytes)should be downloaded before XHR finsihed loading. So can someone explain why only some part of file is downloaded first ?

UPDATE

And i also want to ask when the success call back function is executed, because it will be executed when whole result(response) received from server . So when the success callback function will be executed ? after total download is 7946, OR after the whole data downloaded , ie total downloaded is 277695

Answer 1

The log XHR finished loading... is made by the network monitoring that is part of your console. The point is that this log is made almost at the same time that the true complete transfer has happened, not immediately(!) after the 7946 bytes were read. In short, it is OK like it is. Just use the load event, which triggers immediately(!) after you get the XHR finished message.

Answer 2

Use the XHR "load" event listener:

 xhr.addEventListener("load", transferCompleteCallback);

Read more: https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/Using_XMLHttpRequest