WordPress: Modified addComment Javascript function.

comment-reply-mod.js

var addComment = {
	moveForm : function(commId, parentId, respondId, postId) {
		var t = this, div, comm = t.I(commId), respond = t.I(respondId), cancel = t.I('cancel-comment-reply-link'), parent = t.I('comment_parent'), post = t.I('comment_post_ID');

		if ( ! comm || ! respond || ! cancel || ! parent )
			return;

		t.respondId = respondId;
		postId = postId || false;

		if ( ! t.I('wp-temp-form-div') ) {
			div = document.createElement('div');
			div.id = 'wp-temp-form-div';
			div.style.display = 'none';
			respond.parentNode.insertBefore(div, respond);
		}

                respond.style.display = '';
		comm.parentNode.insertBefore(respond, comm.nextSibling);
		if ( post && postId )
			post.value = postId;
		parent.value = parentId;
		cancel.style.display = '';

		cancel.onclick = function() {
			var t = addComment, temp = t.I('wp-temp-form-div'), respond = t.I(t.respondId);

			if ( ! temp || ! respond )
				return;

			t.I('comment_parent').value = '0';
			temp.parentNode.insertBefore(respond, temp);
			temp.parentNode.removeChild(temp);
                        respond.style.display = 'none';
			this.style.display = 'none';
			this.onclick = null;
			return false;
		};

                var newfocus = 'comment';
                
                if( 'respond_extra' == respondId ){
                    newfocus = 'comment_extra';
                } else if ( 'respond' == respondId ) {
        	    newfocus = 'comment';
                }

                try { t.I( newfocus ).focus(); }
                catch(e) {}


		return false;
	},

	I : function(e) {
		return document.getElementById(e);
	}
};

functions.php

/**
 * Replace the default 'comment-reply.min.js' with the custom 'comment-reply-mod.js'
 *
 * @version 0.0.1
 * @file    functions.php
 */ 
 
function birgire_custom_comment_reply()
{
    wp_deregister_script( 'comment-reply' );
    wp_enqueue_script( 'comment-reply-mod', get_stylesheet_directory_uri() . '/js/comment-reply-mod.js', array(), '1.0.0', true );
}

add_action( 'wp_enqueue_scripts', 'birgire_custom_comment_reply' );


/**
 * Change the default comment form settings
 *
 * @param  array  $defaults
 * @return array  $defaults
 */

function birgire_change_comment_form_defaults( $defaults )
{  
    $change_some_default_settings = array(
        'comment_field'        => '<p class="comment-form-comment"><label for="comment_extra">' . _x( 'Extra Comment', 'noun' ) . '</label> <textarea id="comment_extra" name="comment" cols="45" rows="8" aria-required="true"></textarea></p>',
        'logged_in_as'         => '', 
        'comment_notes_before' => '',
        'comment_notes_after'  => '',
        'id_form'              => 'commentform_extra',
        'id_submit'            => 'submit_extra',
        'title_reply'          => __( 'Leave an Extra Reply' ),
        'title_reply_to'       => __( 'Leave an Extra Reply to %s' ),
        'cancel_reply_link'    => __( 'Cancel Extra reply' ),
        'label_submit'         => __( 'Post an Extra Comment' ),
        'format'               => 'xhtml',
    );
    return wp_parse_args( $change_some_default_settings, $defaults );
}

single.php

// Customize form defaults:
add_action( 'comment_form_defaults', 'birgire_change_comment_form_defaults' );

// Add comments template:
comments_template( '/comments-extra.php', true ); 

// Remove filter:
remove_action( 'comment_form_defaults', 'birgire_change_comment_form_defaults' );