title: On page load event in Chrome extensions
body: <p>I want to check some values in the content of chrome browser page when it completely loaded
  like that</p>
  
  <pre><code>if(document.body.innerText.indexOf("Cat") !=-1)
  </code></pre>
  
  <p>Where and when can I make my check? please give me an clear example
  I read some thing about "Background.html" and "Content script" but I can't do</p>