browser.tabs.onZoomChange.addListener(listener)
browser.tabs.onZoomChange.removeListener(listener)
browser.tabs.onZoomChange.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 listener is registered for this event. Returns true if it is listening, false otherwise.

callback

Function that will be called when this event occurs. The function will be passed the following arguments:

ZoomChangeInfo

object. Information about the zoom event. See the ZoomChangeInfo section for more details.

tabId

integer. ID of the tab that was zoomed.

oldZoomFactor

number. The previous zoom factor.

newZoomFactor

number. The new zoom factor.

zoomSettings

tabs.ZoomSettings. Zoom settings for the tab.

Listen for zoom events and log the info:

function handleZoomed(zoomChangeInfo) {
  console.log(`Tab: ${zoomChangeInfo.tabId} zoomed`);
  console.log(`Old zoom: ${zoomChangeInfo.oldZoomFactor}`);
  console.log(`New zoom: ${zoomChangeInfo.newZoomFactor}`);
}

browser.tabs.onZoomChange.addListener(handleZoomed);