runtime.onMessage

• 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 which will be received by the onMessage listener, use runtime.sendMessage() or (to send a message to a content script) tabs.sendMessage().

Along with the message itself, the listener is passed:

• a sender object giving details about the message sender
• a sendResponse function which it can use 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.

Syntax

browser.runtime.onMessage.addListener(listener)
browser.runtime.onMessage.removeListener(listener)
browser.runtime.onMessage.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 listener 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 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 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 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 Promise.

Browser compatibility

The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

Examples

Simple example

This content script listens for click events in 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 in 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) {
  var 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, 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 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 in 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(function(results) {
    return 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 Window.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
• cookie-bg-picker
• devtools-panels
• export-helpers
• find-across-tabs
• imagify
• mocha-client-tests
• notify-link-clicks-i18n
• proxy-blocker
• store-collected-images
• user-script
• webpack-modules

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