Fired when an extension receives a connection request from a different extension.

To send a message which will be received by the onConnectExternal listener, use runtime.connect(), passing the ID of the recipient in the extensionId parameter.

The listener is passed a runtime.Port object which it can then use to send and receive messages. The Port object also contains a sender property, which is a runtime.MessageSender object, and which the recipient can use to check the sender's ID.



Events have three functions:


Adds a listener to this event.


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


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

addListener syntax



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


A runtime.Port object connecting the current script to the other extension it is connecting to.

Browser compatibility

BCD tables only load in the browser


In this example the extension Hansel connects to the extension Gretel:

console.log("connecting to Gretel");
let myPort = browser.runtime.connect(

myPort.onMessage.addListener((message) => {
  console.log(`From Gretel: ${message.content}`);

browser.browserAction.onClicked.addListener(() => {
  myPort.postMessage({content: "Hello from Hansel"});

Gretel listens for the connection and checks that the sender is really Hansel:

let portFromHansel;

browser.runtime.onConnectExternal.addListener((port) => {
  if ( === "") {
    console.log("connection attempt from Hansel");
    portFromHansel = port;
    portFromHansel.onMessage.addListener((message) => {
      console.log(`From Hansel: ${message.content}`);

browser.browserAction.onClicked.addListener(() => {
   portFromHansel.postMessage({content: "Message from Gretel"});

Note: This API is based on Chromium's chrome.runtime API. This documentation is derived from runtime.json in the Chromium code.

Microsoft Edge compatibility data is supplied by Microsoft Corporation and is included here under the Creative Commons Attribution 3.0 United States License.