CoCalc -- index.md

GitHub Repository: mohamedkhallouq/content
Path: blob/main/files/en-us/mozilla/add-ons/webextensions/api/runtime/onmessage/index.md
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---

title: runtime.onMessage
slug: Mozilla/Add-ons/WebExtensions/API/runtime/onMessage
page-type: webextension-api-event
tags:
  - API
  - Add-ons
  - Event
  - Extensions
  - Non-standard
  - Reference
  - WebExtensions
  - onmessage
  - runtime
browser-compat: webextensions.api.runtime.onMessage

---

Use this event to listen for messages from another part of your extension.

Some example use cases are:

in a content script, to listen for messages from a background script.
in a background script, to listen for messages from a content script.
in an options page or popup script, to listen for messages from a background script.
in a background script, to listen for messages from an options page or popup script.

To send a message that is received by the onMessage() listener, use {{WebExtAPIRef("runtime.sendMessage()")}} or (to send a message to a content script) {{WebExtAPIRef("tabs.sendMessage()")}}.

Note: Avoid creating multiple onMessage() listeners for the same type of message, because the order in which multiple listeners will fire is not guaranteed.
If you want to guarantee the delivery of a message to a specific end point, use the connection-based approach to exchange messages.

Along with the message itself, the listener is passed:

a sender object giving details about the message sender.
a sendResponse() function that can be used to send a response back to the sender.

You can send a synchronous response to the message by calling the sendResponse() function inside your listener. See an example.

To send an asynchronous response, there are two options:

return true from the event listener. This keeps the sendResponse() function valid after the listener returns, so you can call it later. See an example.
return a Promise from the event listener, and resolve when you have the response (or reject it in case of an error). See an example.

Note: You can also use a connection-based approach to exchange messages.

Syntax

browser.runtime.onMessage.addListener(listener)
browser.runtime.onMessage.removeListener(listener)
browser.runtime.onMessage.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 at least one listener is registered for this event. Returns true if it is listening, false otherwise.

addListener syntax

Parameters

listener
- : A callback function that will be called when this event occurs. The function will be passed the following arguments:
  - message
    - : object. The message itself. This is a serializable object (see Data cloning algorithm).
  - 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 a single argument, which may be any serializable object (see Data cloning algorithm). This argument is passed back to the message sender.
      If you have more than one onMessage() 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:
      - either keep a reference to the sendResponse() argument and return true from the listener function. You will then be able to call sendResponse() after the listener function has returned.
      - or return a {{jsxref("Promise")}} from the listener function and resolve the promise when the response is ready. This is a preferred way.
  The listener function can return either a Boolean or a {{jsxref("Promise")}}.
  Note: If you pass an async function to addListener(), the listener will return a Promise for every message it receives, preventing other listeners from responding:
  // don't do this browser.runtime.onMessage.addListener( async (data, sender) => { if (data.type === 'handle_me') { return 'done'; } } );
  If you only want the listener to respond to messages of a certain type, you must define the listener as a non-async function, and return a Promise only for the messages the listener is meant to respond to — and otherwise return false or undefined:
  browser.runtime.onMessage.addListener( (data, sender) => { if (data.type === 'handle_me') { return Promise.resolve('done'); } return false; } );

Browser compatibility

Examples

Simple example

This content script listens for click events on the web page. If the click was on a link, it messages the background page with the target URL:

// content-script.js

window.addEventListener("click", notifyExtension);

function notifyExtension(e) {
  if (e.target.tagName !== "A") {
    return;
  }
  browser.runtime.sendMessage({"url": e.target.href});
}

The background script listens for these messages and displays a notification using the notifications API:

// background-script.js

browser.runtime.onMessage.addListener(notify);

function notify(message) {
  browser.notifications.create({
    "type": "basic",
    "iconUrl": browser.extension.getURL("link.png"),
    "title": "You clicked a link!",
    "message": message.url
  });
}

Sending a synchronous response

This content script sends a message to the background script when the user clicks on the page. It also logs any response sent by the background script:

// content-script.js

function handleResponse(message) {
  console.log(`background script sent a response: ${message.response}`);
}

function handleError(error) {
  console.log(`Error: ${error}`);
}

function sendMessage(e) {
  const sending = browser.runtime.sendMessage({content: "message from the content script"});
  sending.then(handleResponse, handleError);
}

window.addEventListener("click", sendMessage);

Here is a version of the corresponding background script, that sends a response synchronously, from inside in the listener:

// background-script.js

function handleMessage(request, sender, sendResponse) {
  console.log(`content script sent a message: ${request.content}`);
  sendResponse({response: "response from background script"});
}

browser.runtime.onMessage.addListener(handleMessage);

And here is another version which uses {{jsxref("Promise.resolve()")}}:

// background-script.js

function handleMessage(request, sender, sendResponse) {
  console.log(`content script sent a message: ${request.content}`);
  return Promise.resolve({response: "response from background script"});
}

browser.runtime.onMessage.addListener(handleMessage);

Sending an asynchronous response using sendResponse

Here is an alternative version of the background script from the previous example. It sends a response asynchronously after the listener has returned. Note return true; in the listener: this tells the browser that you intend to use the sendResponse argument after the listener has returned.

// background-script.js

function handleMessage(request, sender, sendResponse) {
  console.log(`content script sent a message: ${request.content}`);
  setTimeout(() => {
    sendResponse({response: "async response from background script"});
  }, 1000);
  return true;
}

browser.runtime.onMessage.addListener(handleMessage);

Sending an asynchronous response using a Promise

This content script gets the first <a> link on the page and sends a message asking if the link's location is bookmarked. It expects to get a Boolean response (true if the location is bookmarked, false otherwise):

// content-script.js

const firstLink = document.querySelector("a");

function handleResponse(isBookmarked) {
  if (isBookmarked) {
    firstLink.classList.add("bookmarked");
  }
}

browser.runtime.sendMessage({
  url: firstLink.href
}).then(handleResponse);

Here is the background script. It uses {{WebExtAPIRef("bookmarks.search()")}} to see if the link is bookmarked, which returns a {{jsxref("Promise")}}:

// background-script.js

function isBookmarked(message, sender, response) {
  return browser.bookmarks.search({
    url: message.url
  }).then((results) => results.length > 0);
}

browser.runtime.onMessage.addListener(isBookmarked);

If the asynchronous handler doesn't return a Promise, you can explicitly construct a promise. This rather contrived example sends a response after a 1-second delay, using setTimeout():

// background-script.js

function handleMessage(request, sender, sendResponse) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve({response: "async response from background script"});
    }, 1000);
  });
}

browser.runtime.onMessage.addListener(handleMessage);

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