<div id="comment1">
    <p class="comment">Hello World</p>
    <button class="edit">edit</button>
 </div>

 <div id="comment2">
    <p class="comment">Hello World 123</p>
    <button class="edit">edit</button>
 </div>

 <div id="comment3">
     <p class="comment">Hello World 456</p>
     <button class="edit">edit</button>
 </div>

 <br>
 <textarea class="commentArea"></textarea>

$('.edit').click(function(e){
   var target = $(e.target),
     prevComment =  target.parent().children().filter('p.comment'),
     commentText = prevComment.text();

     $('.commentArea').text(commentText);
});