ProgressEvent: loaded property

The ProgressEvent.loaded read-only property is a number indicating the size of the data already transmitted or processed. The progress ratio can be calculated by dividing the value of this property by ProgressEvent.total.

For ProgressEvents dispatched by the browser in HTTP messages, the value refers to the amount of bytes of a resource that are completed, and is derived from the Content-Length header. 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.

In a ProgressEvent you create yourself, you can assign any numeric value to loaded that represents the amount of work completed relative to the total value.