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 {{WebExtAPIRef("runtime.sendMessage()")}}, passing the ID of the recipient in the extensionId parameter.

Along with the message, the listener is passed:

• a sender object with 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.

See {{WebExtAPIRef("runtime.onMessage")}} for additional information on receiving messages and sending responses, as well as examples of the various options for sending responses.

Syntax

browser.runtime.onMessageExternal.addListener()
browser.runtime.onMessageExternal.removeListener(listener)
browser.runtime.onMessageExternal.hasListener(listener)

Events have three functions:

• addListener(listener)
  • : 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

• listener
  • : The function called when this event occurs. The function is passed these arguments:
    • message
      • : object. The message. This is a JSON-ifiable object.
    • sender
      • : A {{WebExtAPIRef('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 one argument, which is a 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 can send a response.

        To send a response synchronously, call sendResponse() before the listener function returns.

        To send a response asynchronously, use one of these options:

        • Return a {{jsxref("Promise")}} from the listener function and resolve the promise when the response is ready. This is the preferred approach.

        • Keep a reference to the sendResponse() argument and return true from the listener function. You then call sendResponse() after the listener function returns.

          [!NOTE] Promise as a return value is not supported in Chrome until Chrome bug 1185241 is resolved. As an alternative, return true and use sendResponse as described for runtime.onMessage.

Examples

In this example, the extension "[email protected]" sends a message to the extension "[email protected]":

// sender: browser.runtime.id === "[email protected]"

// Send a message to the extension whose ID is "[email protected]"
browser.runtime.sendMessage("[email protected]", "my message");

// recipient: browser.runtime.id === "[email protected]"

function handleMessage(message, sender) {
  // check that the message is from "[email protected]"
  if (sender.id === "[email protected]") {
    // process message
  }
}

browser.runtime.onMessageExternal.addListener(handleMessage);

{{WebExtExamples}}

Browser compatibility

{{Compat}}

[!NOTE] This API is based on Chromium's chrome.runtime API. This documentation is derived from runtime.json in the Chromium code.