how to get the comment ID in the front end when the REPLY button is clicked?

Here are few ideas:

• It’s possible to inject custom HTML or data attributes via the comment_reply_link filter, but I think it would be better to keep it unobtrusive.

• One could try to get the comment parent value from the reply comment form:

  <input type="hidden" name="comment_parent" id='comment_parent' value="0" />
  

  after it has been updated with the corresponding value.

• Another approach would be to scan for IDs in the HTML of the comments list:

  Generated by the Walker_Comment::html5_comment():

  <li id="comment-34" class="comment even thread-even depth-1">
      <article id="div-comment-34" class="comment-body">
      ... cut...
  

  Generated by the Walker_Comment::comment():

  <li class="comment even thread-even depth-1 parent" id="comment-34">
      <div id="div-comment-34" class="comment-body">
      ... cut...
  

  One could e.g. search for both cases, but it’s possible for the user to override these with a custom output.

  Also note the depth information here.

• One could try to parse the onclick attribute of the replyto link:

  onclick='return addComment.moveForm( "div-comment-34", "34", "respond", "123" )'