The reportId property of the HIDInputReportEvent interface returns the one-byte identification prefix for this report, or 0 if the HID interface does not use report IDs.

A one-byte identification prefix.

In the following example the reportId of an incoming input report is logged to the console.

device.addEventListener("inputreport", (event) => {
  const { data, device, reportId } = event;
  console.log(reportId);
});