The read-only property is an unsigned 64-bit integer value indicating the total size of the data being processed or transmitted. In the case of an HTTP transmission, this is the size of the body of the message (the Content-Length), and does not include headers and other overhead.

If the event's lengthComputable property is false, this value is meaningless and should be ignored.


let value =;


Specification Status Comment
The definition of 'ProgressEvent.lengthComputable' in that specification.
Living Standard

Browser compatibility

BCD tables only load in the browser

See also