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runtime.onMessage

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

Some example use cases are:

  • in acontent script to listen for messages from abackground script.
  • in a background script to listen for messages from a content script.
  • in anoptions 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 theonMessage() listener, useruntime.sendMessage() or (to send a message to a content script)tabs.sendMessage().

Note:Avoid creating multipleonMessage() listeners for the same type of message because the order in which multiple listeners fire is not guaranteed.

If you want to guarantee the delivery of a message to a specific endpoint, use theconnection-based approach to exchange messages.

Along with the message itself, the listener is passed:

  • asender object with details about the message sender.
  • asendResponse() function that can be used to send a response back to the sender.

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

To send an asynchronous response, there are two options:

Syntax

js
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. Thelistener argument is the listener to remove.

hasListener(listener)

Checks whether at least one listener is registered for this event. Returnstrue 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 (seeData cloning algorithm).

sender

Aruntime.MessageSender object representing the sender of the message.

sendResponse

A function to call, at most once, to send a response to themessage. The function takes one argument: any serializable object (seeData cloning algorithm). This argument is passed back to the message sender.

If you have more than oneonMessage() listener in the same document, then only one can send a response.

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

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

  • Return aPromise from the listener function and resolve the promise when the response is ready. This is the preferred approach.

  • Keep a reference to thesendResponse() argument and returntrue from the listener function. You then callsendResponse() after the listener function returns.

    Note:Promise as a return value is not supported in Chrome untilChrome bug 1185241 is resolved. As an alternative,return true and use sendResponse.

Thelistener function can return either a Boolean or aPromise.

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

js
// don't do thisbrowser.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:

js
// content-script.jswindow.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 thenotifications API:

js
// background-script.jsbrowser.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:

js
// content-script.jsfunction 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:

js
// background-script.jsfunction 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 usesPromise.resolve():

js
// background-script.jsfunction 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. Notereturn true; in the listener: this tells the browser that you intend to use thesendResponse argument after the listener returns.

js
// background-script.jsfunction 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);

Warning:Do not prependasync to the function. Prependingasync changes the meaning tosending an asynchronous response using a promise, which is effectively the same assendResponse(true).

Sending an asynchronous response using a Promise

Note:Promise as a return value is not supported in Chrome untilChrome bug 1185241 is resolved. As an alternative,return true and usesendResponse.

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):

js
// content-script.jsconst 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 usesbookmarks.search() to see if the link is bookmarked, which returns aPromise:

js
// background-script.jsfunction 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, usingsetTimeout():

js
// background-script.jsfunction handleMessage(request, sender, sendResponse) {  return new Promise((resolve) => {    setTimeout(() => {      resolve({ response: "async response from background script" });    }, 1000);  });}browser.runtime.onMessage.addListener(handleMessage);

Example extensions

Browser compatibility

Note:This API is based on Chromium'schrome.runtime API. This documentation is derived fromruntime.json in the Chromium code.

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