ProgressEvent: loaded property

The ProgressEvent.loaded read-only property is a 64-bit unsigned integer indicating the size, in bytes, of the data already transmitted or processed. The ratio can be calculated by dividing ProgressEvent.total by the value of this property. When downloading a resource using HTTP, this only counts the body of the HTTP message, and doesn't include headers and other overhead.

Note that for compressed requests of unknown total size, loaded might contain the size of the compressed, or decompressed, data, depending on the browser. As of 2024, it contains the size of the compressed data in Firefox, and the size of the uncompressed data in Chrome.