get element by class for chrome

Question

I am trying to add this code into a chrome extension to alert me when a chatbox is available. Its in a div with class name shoutbox as of now it doesnt work.

function Detection(){
    if(document.getElementsByClassName("shoutbox")!=null){
      alert('Chat is available.')
    }
}

Detection();

Updated Code: the page loads and the alert dialog never appears.

function Detection(){
    if(document.getElementsByClassName("shoutbox").length > 0){
        alert('Chat is available.')
    }
}

window.onload = Detection;

Answer 1

== null won't detect an empty array (no results). You could write

if(document.getElementsByClassName("shoutbox").length > 0){
  alert('Chat is available.')
}