Non-standard: This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
 
This property specifies a callback function to be run repeatedly while the operation represented by a FileRequest object is in progress.
 Each time the function callback is called, it gets an object as its first parameter. Those objects contain two properties:
  loaded  -  
A number representing the current amount of bytes processed by the operation.
   total  -  
A number representing the total amount of bytes that will be processed by the operation.
   
 
request.onprogress = function (status) {
  var progress = document.querySelector('progress');
  progress.value = status.loaded;
  progress.max   = status.total;
}
 
Not part of any current specification.