runtime.onMessage

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

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

message

object. The message. This is a serializable object (see Data cloning algorithm).

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 one argument: 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 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 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.

The listener function can return either a Boolean or a Promise.

Note: If you pass an async function to addListener(), the listener returns a Promise for every message it receives, preventing other listeners from responding:

js

// don't do this
browser.runtime.onMessage.addListener(async (data, sender) => {
  if (data.type === "handle_me") {
    return "done";
  }
});

Suppose you only want the listener to respond to messages of a specific type. In that case, 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:

js

browser.runtime.onMessage.addListener((data, sender) => {
  if (data.type === "handle_me") {
    return Promise.resolve("done");
  }
  return false;
});

Examples

Simple example

This content script listens for click events on the web page. If the click is 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 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 that uses 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 returns. Note return true; in the listener: this tells the browser that you intend to use the sendResponse argument after the listener returns.

// 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 bookmarks.search() to see if the link is bookmarked, which returns a 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);

Example extensions

• beastify
• content-script-register
• cookie-bg-picker
• devtools-panels
• export-helpers
• find-across-tabs
• imagify
• mocha-client-tests
• notify-link-clicks-i18n
• store-collected-images
• user-script-register
• userScripts-mv3
• webpack-modules

Browser compatibility