createComment() creates a new comment node, and returns it.
Syntax
CommentNode = document.createComment(data)
Parameters
data- A string containing the data to be added to the Comment.
Example
var docu = new DOMParser().parseFromString('<xml></xml>', "application/xml");
var comment = docu.createComment('This is a not-so-secret comment in your document');
docu.getElementsByTagName('xml')[0].appendChild(comment);
alert(new XMLSerializer().serializeToString(docu));
// Displays: <xml><!--This is a not-so-secret comment in your document--></xml>