runtime.onMessageExternal

Use this event to listen for messages from other extensions or web pages.

By default, an extension can receive messages from any other extension. However, the externally_connectable manifest key can be used to limit communication to specific extensions and enable communication with websites.

To send a message that is received by the onMessageExternal listener, use runtime.sendMessage(), passing the ID of the recipient in the extensionId parameter.

Along with the message itself, the listener is passed:

a sender object giving details about the message sender
a sendResponse function that the listener can use to send a response back to the sender.

This API can't be used in a content script.

Syntax

browser.runtime.onMessageExternal.addListener()
browser.runtime.onMessageExternal.removeListener(listener)
browser.runtime.onMessageExternal.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): Checks whether a listener is registered for this event. Returns true if it is listening, false otherwise.

addListener syntax

Parameters

function

A callback function that is called when this event occurs. The function is passed these arguments:

message

object. The message itself. This is a JSON-ifiable object.

sender

A runtime.MessageSender object representing the sender of the message.

sendResponse

A function to call, at most once, to send a response to the message. The function takes a single argument, which may be any JSON-ifiable object. This argument is passed back to the message sender.

If you have more than one onMessageExternal listener in the same document, then only one may send a response.

To send a response synchronously, call sendResponse before the listener function returns. To send a response asynchronously, do one of these:

keep a reference to the sendResponse argument and return true from the listener function. You can then call sendResponse after the listener function has returned.
return a Promise from the listener function and resolve the promise when the response is ready.

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`onMessageExternal`	26	79	54	?	15	15.4 Since Safari 15.4, this event is also fired in response to a message from webpages allowed in `externally_connectable`. 14 This event is only fired in response to a message from an extension's containing app, not webpages nor other extensions.	?	?	54	?	15.4 Since Safari 15.4, this event is also fired in response to a message from webpages allowed in `externally_connectable`. 15 This event is only fired in response to a message from an extension's containing app, not webpages nor other extensions.	?
`return_promise`	No	No	Yes	?	No	15.4	?	?	Yes	?	15.4	?

Examples

In this example the extension "blue@mozilla.org" sends a message to the extension "red@mozilla.org":

// sender: browser.runtime.id === "blue@mozilla.org"

// Send a message to the extension whose ID is "red@mozilla.org"
browser.runtime.sendMessage(
    "red@mozilla.org",
    "my message"
  );

// recipient: browser.runtime.id === "red@mozilla.org"

function handleMessage(message, sender) {
  // check that the message is from "blue@mozilla.org"
  if (sender.id === "blue@mozilla.org") {
    // process message
  }
}

browser.runtime.onMessageExternal.addListener(handleMessage);

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

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/onMessageExternal