JavaScript: The Definitive Guide, 6th Edition

Name

ProgressEvent — downloading, uploading, or file reading progress

Inherits from

Event

Synopsis

The ApplicationCache, FileReader, and (level 2) XMLHttpRequest object all fire Progress events to inform interested applications of the progress of a data transfer process such as a network download or upload or a file read. Events of this sort are known generically as Progress events, but only one such event actually has the name “progress.” Other Progress events fired by FileReader and XMLHttpRequest are loadstart, load, loadend, error and abort. XMLHttpRequest also fires a timeout Progress event. ApplicationCache fires a number of events, but only the one named “progress” is a Progress event of the type described here.

Progress events are triggered in a sequence that begins with a loadstart event and always ends with a loadend event. The event immediately before loadend will be load, error, or abort, depending on whether the data transfer operation succeeded and if not, how it failed. Zero or more progress events (with the actual event name “progress”) are triggered between the initial loadstart and the final two events. (The ApplicationCache object fires a different sequence of events, but the progress event it fires as part of its cache update process is a Progress event.)

Event handlers for Progress events are passed a ProgressEvent object that specifies how many bytes of data have been transferred. This ProgressEvent object is unrelated to the HTML <progress> element described in ...