chrome extension message from background.js to content.js with response, TypeError cannot read property

Question

Hello I am new to chrome extensions and I'm trying to make a extension. I want the background.js to send a request to content.js every time the user goes to another tab and for that I have this code inside background.js

chrome.tabs.onActivated.addListener(function() {
    chrome.tabs.query({active: true, currentWindow: true}, function(tabs) {
      chrome.tabs.sendMessage(tabs[0].id, {greeting: "update"}, function(response) {
          console.log(response.proto);
      });
    });
});

and in content.js I have this code to receive the request and send a response:

chrome.runtime.onMessage.addListener(
    function(request, sender, sendResponse) {
        console.log(request.greeting);
        var protocol = window.location.protocol;
        sendResponse({ proto: protocol });
    }
);

I get this error in the background console:

Error in event handler for (unknown): TypeError: Cannot read property 'proto' of undefined
    at background.js:6:32

and I get no error in the normal console. I have used this documentation:

https://developer.chrome.com/apps/messaging

and

https://developer.chrome.com/extensions/tabs#event-onActivated

What am I doing wrong?

Answer 1

content.js only runs in tabs whose url defined in the manifest.json file. if the url of the active tab does not match these urls ,the background.js gets none.