extensionId Optional

string. The ID of the extension to send the message to. Include this to send the message to a different extension. If the intended recipient has set an ID explicitly using the browser_specific_settings key in manifest.json, then extensionId should have that value. Otherwise it should have the ID that was generated for the intended recipient.

If extensionId is omitted, the message is sent to your extension.

message

any. An object that can be structured clone serialized (see Data cloning algorithm).

options Optional

object.

includeTlsChannelId Optional

boolean. Whether the TLS channel ID will be passed into runtime.onMessageExternal for processes that are listening for the connection event.

This option is only supported in Chromium-based browsers.

Depending on the arguments it is given, this API is sometimes ambiguous. The following rules are used:

• if one argument is given, it is the message to send, and the message will be sent internally.
• if two arguments are given:
  • the arguments are interpreted as (message, options), and the message is sent internally, if the second argument is any of the following:
    1. a valid options object (meaning, it is an object which contains only the properties of options that the browser supports)
    2. null
    3. undefined
  • otherwise, the arguments are interpreted as (extensionId, message). The message will be sent to the extension identified by extensionId.
• if three arguments are given, the arguments are interpreted as (extensionId, message, options). The message will be sent to the extension identified by extensionId.

Note that before Firefox 55, the rules were different in the 2-argument case. Under the old rules, if the first argument was a string, it was treated as the extensionId, with the message as the second argument. This meant that if you called sendMessage() with arguments like ("my-message", {}), then it would send an empty message to the extension identified by "my-message". Under the new rules, with these arguments you would send the message "my-message" internally, with an empty options object.

A Promise. If the receiver sent a response, this will be fulfilled with the response. Otherwise it will be fulfilled with no arguments. If an error occurs while connecting to the extension, the promise will be rejected with an error message.

Here's a content script that sends a message to the background script when the user clicks the content window. The message payload is {greeting: "Greeting from the content script"}, and the sender also expects to get a response, which is handled in the handleResponse function:

// content-script.js

function handleResponse(message) {
  console.log(`Message from the background script: ${message.response}`);
}

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

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

window.addEventListener("click", notifyBackgroundPage);

The corresponding background script looks like this:

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

browser.runtime.onMessage.addListener(handleMessage);

• content-script-register
• devtools-panels
• export-helpers
• find-across-tabs
• mocha-client-tests
• notify-link-clicks-i18n
• store-collected-images
• user-script-register
• webpack-modules