XMLHttpRequest Upload Progress Events

    var xhr = new XMLHttpRequest();
    xhr.upload.addEventListener('loadstart', function(e) {
      // When the request starts.
    });
    xhr.upload.addEventListener('progress', function(e) {
      // While sending and loading data.
    });
    xhr.upload.addEventListener('load', function(e) {
      // When the request has *successfully* completed.
      // Even if the server hasn't responded that it finished.
    });
    xhr.upload.addEventListener('loadend', function(e) {
      // When the request has completed (either in success or failure).
      // Just like 'load', even if the server hasn't 
      // responded that it finished processing the request.
    });
    xhr.upload.addEventListener('error', function(e) {
      // When the request has failed.
    });
    xhr.upload.addEventListener('abort', function(e) {
      // When the request has been aborted. 
      // For instance, by invoking the abort() method.
    });
    xhr.upload.addEventListener('timeout', function(e) {
      // When the author specified timeout has passed 
      // before the request could complete.
    });
    // notice that the event handler is on xhr and not xhr.upload
    xhr.addEventListener('readystatechange', function(e) {
      if( this.readyState === 4 ) {
        // the transfer has completed and the server closed the connection.
      }
    });