chrome.extension.onRequestExternal.addListener(function(
  request,      // optional any
  sender,       // runtime.MessageSender
  sendResponse, // function
) { })
chrome.extension.onRequestExternal.removeListener(listener)
chrome.extension.onRequestExternal.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:

request

any. The request sent by the calling script.

sender

runtime.MessageSender.

sendResponse

function. Function to call when you have a response. The argument should be any JSON-ifiable object, or undefined if there is no response.