ProgressEvent interface represents events measuring progress of an underlying process, like an HTTP request (for an
XMLHttpRequest, or the loading of the underlying resource of an
Boolean flag indicating if the total work to be done, and the amount of work already done, by the underlying process is calculable. In other words, it tells if the progress is measurable or not.
unsigned long long representing the amount of work already performed by the underlying process. The ratio of work done can be calculated with the property and
ProgressEvent.total. When downloading a resource using HTTP, this only represent the part of the content itself, not headers and other overhead.