I have Copied Below code but its not working in my application. Why?

Question

this code i have copied from here Stack OverFlow it is not working in my application BUT IT IS WORKING IN jsfiddle. It is not performing any action in Application.

<textarea name="comment_text" id="comment_text"  class="Post_Description_Text"></textarea>

$(".Post_Description_Text").keydown(function(e){
     if (e.keyCode == 13 && !e.shiftKey){ 
          e.preventDefault();return false;
     }   
});