browser.downloads.onErased.addListener(listener)
browser.downloads.onErased.removeListener(listener)
browser.downloads.onErased.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:

downloadId

An integer representing the id of the downloads.DownloadItem that was erased.

Add a listener for onErased events, then erase the most recent download:

function handleErased(item) {
  console.log(`Erased: ${item}`);
}

browser.downloads.onErased.addListener(handleErased);

let erasing = browser.downloads.erase({
  limit: 1,
  orderBy: ["-startTime"]
});