browser.downloads.onChanged.addListener(listener)
browser.downloads.onChanged.removeListener(listener)
browser.downloads.onChanged.hasListener(listener)

Events have three functions:

addListener(callback)

Adds a listener to this event.

removeListener(listener)

Stop listening to this event. The listener argument is the listener to remove.

hasListener(listener)

Check whether a given listener is registered for this event. Returns true if it is listening, false otherwise.

callback

A callback function that will be called when this event occurs. This function will be passed the following arguments:

downloadDelta

An object representing the downloads.DownloadItem object that changed, and the status of all the properties that changed in it. See the downloadDelta section for more details.

The downloadDelta object has the following properties available:

id

An integer representing the id of the downloads.DownloadItem that changed.

url Optional

A downloads.StringDelta object describing a change in a downloads.DownloadItem's url.

filename Optional

A downloads.StringDelta object describing a change in a downloads.DownloadItem's filename.

danger Optional

A downloads.StringDelta object describing a change in a downloads.DownloadItem's danger.

mime Optional

A downloads.StringDelta object describing a change in a downloads.DownloadItem's mime.

startTime Optional

A downloads.StringDelta object describing a change in a downloads.DownloadItem's startTime.

endTime Optional

A downloads.StringDelta object describing a change in a downloads.DownloadItem's endTime.

state Optional

A downloads.StringDelta object describing a change in a downloads.DownloadItem's state.

canResume Optional

A downloads.BooleanDelta object describing a change in a downloads.DownloadItem's canResume status.

paused Optional

A downloads.BooleanDelta object describing a change in a downloads.DownloadItem's paused status.

error Optional

A downloads.StringDelta object describing a change in a downloads.DownloadItem's error status.

totalBytes Optional

A downloads.DoubleDelta object describing a change in a downloads.DownloadItem's totalBytes.

fileSize Optional

A downloads.DoubleDelta object describing a change in a downloads.DownloadItem's fileSize.

exists Optional

A downloads.BooleanDelta object describing a change in a downloads.DownloadItem's exists status.

Log a message when downloads complete:

function handleChanged(delta) {
  if (delta.state && delta.state.current === "complete") {
    console.log(`Download ${delta.id} has completed.`);
  }
}

browser.downloads.onChanged.addListener(handleChanged);