The progress event of the BackgroundFetchRegistration interface thrown when the associated background fetch progresses.

Practically, this event is fired when any of the following properties will return a new value:

• uploaded,
• downloaded,
• result, or
• failureReason.

Use the event name in methods like addEventListener(), or set the onprogress event property on the relevant BackgroundFetchRegistration object.

addEventListener("progress", (event) => { })
onprogress = (event) => { }

A generic Event with no added properties.

The following example demonstrates how to log the progress of a download. The code first checks that a downloadTotal was provided when the background fetch was registered. This is then used to calculate the percentage, based on the downloaded property.

bgFetch.addEventListener("progress", () => {
  if (!bgFetch.downloadTotal) return;
  const percent = Math.round(
    (bgFetch.downloaded / bgFetch.downloadTotal) * 100,
  );
  console.log(`Download progress: ${percent}%`);
});