Example of basic JS messaging for Chrome extensions

messaging.js

// In the extension JS
chrome.tabs.query({active: true, currentWindow: true}, function(tabs) { // Fetch the current tab
  chrome.tabs.sendMessage(tabs[0].id, {message: "preach", preachText: usrMessage});
});

// In the context script
chrome.runtime.onMessage.addListener(
  function(request, sender, sendResponse) {
    if (request.message == "preach"){ // Filter out other messages
      alert(request.preachText, 5000);
    }
});