inject.js

// This helps avoid conflicts in case we inject
// this script on the same page multiple times
// without reloading.
var injected = injected || (function(){

  // An object that will contain the "methods"
  // we can use from our event script.
  var methods = {};

  // This method will eventually return
  // background colors from the current page.
  methods.getBgColors = function(){
    var nodes = document.querySelectorAll('*');
    return nodes.length;
  };

  // This tells the script to listen for
  // messages from our extension.
  chrome.runtime.onMessage.addListener(function (request, sender, sendResponse) {
    var data = {};
    // If the method the extension has requested
    // exists, call it and assign its response
    // to data.
    if (methods.hasOwnProperty(request.method))
      data = methods[request.method]();
    // Send the response back to our extension.
    sendResponse({ data: data });
    return true;
  });

  return true;
})();