jpcomm_pagemod_js.js

// Listen for messages coming from main.js
onMessage = function onMessage(msg){
  // since any number of commands might be passed via the message,
  // we need to look at the msg.type and process accordingly.
  if(msg.type == "color_change"){
      // in this case, the message is telling us to change the 
      // documents background color.
      document.body.style.backgroundColor = msg.color;
      // now that we have done that, tell main.js the task was 
      // completed.
      postMessage({
         "type": "color_change_complete",
         "status": "ok",
         "color": msg.color
      });
  }else{
      // We've encounted an message that we don't know how to
      // handle...
      console.log("unknown message type...");   
  }
}