How to check if Tab in google chrome lost focus?

Question

Google Chrome opens up with PID ,and every tab opened get a PID but when you  switch between tabs , the currently selected tab gets the primary PID , so my question is how can i know that a specific tab loses focus ?

thanx
yossi

Answer 1

this is JS source code: http://331542.esy.es/game/js/index.js

Answer 2

http://331542.esy.es/game/js/index.js