I am developing a Chrome extension. I want to pass a message from the contentScript.js to my popup.js file. This should happen when the contentScript.js loads. From what I have read, I use " chrome.runtime.sendMessage" to do this. I am not sure how to actually receive it in the popup.js file. I have read that you need to use "chrome.runtime.onMessage.addListener", however it does not work.

contentScript.js

window.onload = function() {  
    var markup = document.documentElement.innerHTML;
    alert(markup);

    chrome.runtime.sendMessage({a:"b"});

    //This alert fires.
    alert("Content sent");
};

popup.js

    chrome.runtime.onMessage.addListener(
       function(request, sender, sendResponse) {
        //This alert never fires. 
        alert("Message Received!"); 
       }
   );

There are no errors received in the console, nor the extensions page. The message is either never sent, or is sent on not received. What am I missing?