javascript - How to get progress from XMLHttpRequest

Question

Is it possible to get the progress of an XMLHttpRequest (bytes uploaded, bytes downloaded)?

This would be useful to show a progress bar when the user is uploading a large file. The standard API doesn't seem to support it, but maybe there's some non-standard extension in any of the browsers out there? It seems like a pretty obvious feature to have after all, since the client knows how many bytes were uploaded/downloaded.

note: I'm aware of the "poll the server for progress" alternative (it's what I'm doing right now). the main problem with this (other than the complicated server-side code) is that typically, while uploading a big file, the user's connection is completely hosed, because most ISPs offer poor upstream. So making extra requests is not as responsive as I'd hoped. I was hoping there'd be a way (maybe non-standard) to get this information, which the browser has at all times.

Answer 1

For the bytes uploaded it is quite easy. Just monitor the xhr.upload.onprogress event. The browser knows the size of the files it has to upload and the size of the uploaded data, so it can provide the progress info.

For the bytes downloaded (when getting the info with xhr.responseText), it is a little bit more difficult, because the browser doesn't know how many bytes will be sent in the server request. The only thing that the browser knows in this case is the size of the bytes it is receiving.

There is a solution for this, it's sufficient to set a Content-Length header on the server script, in order to get the total size of the bytes the browser is going to receive.

For more go to https://developer.mozilla.org/en/Using_XMLHttpRequest .

Example: My server script reads a zip file (it takes 5 seconds):

$filesize=filesize('test.zip');

header("Content-Length: " . $filesize); // set header length
// if the headers is not set then the evt.loaded will be 0
readfile('test.zip');
exit 0;

Now I can monitor the download process of the server script, because I know it's total length:

function updateProgress(evt) 
{
   if (evt.lengthComputable) 
   {  // evt.loaded the bytes the browser received
      // evt.total the total bytes set by the header
      // jQuery UI progress bar to show the progress on screen
     var percentComplete = (evt.loaded / evt.total) * 100;  
     $('#progressbar').progressbar( "option", "value", percentComplete );
   } 
}   
function sendreq(evt) 
{  
    var req = new XMLHttpRequest(); 
    $('#progressbar').progressbar();    
    req.onprogress = updateProgress;
    req.open('GET', 'test.php', true);  
    req.onreadystatechange = function (aEvt) {  
        if (req.readyState == 4) 
        {  
             //run any callback here
        }  
    };  
    req.send(); 
}