browser.captivePortal.onConnectivityAvailable.addListener(callback)
browser.captivePortal.onConnectivityAvailable.removeListener(listener)
browser.captivePortal.onConnectivityAvailable.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 is called when this event occurs. The function is passed the following arguments:

status

string The status of the service, being one of captive if there is an unlocked captive portal present or clear if no captive portal is detected.

Handle a change user's ability to connect to the internet:

function handleConnectivity(connectivityInfo) {
  console.log(`The captive portal status: ${connectivityInfo.status}`);
}

browser.captivePortal.onConnectivityAvailable.addListener(handleConnectivity);