chromium/chrome/test/data/extensions/api_test/tabs/send_message/echo.js

// Copyright 2021 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// A simple onMessage listener that responds to "ping" messages with a "pong"
// message.
chrome.runtime.onMessage.addListener((request, sender, sendResponse) => {
  const extensionOrigin = new URL(chrome.runtime.getURL('')).origin;
  let response = 'pong';
  if (request != 'ping' || sender.origin != extensionOrigin)
    response = `Unexpected message "${request}" from origin "${sender.origin}"`;
  sendResponse(response);
});