ok, I found out how. you simply create a template with the error name inside your theme. example 404.tpl
hopefully, this might help someone.

I solved my problem, but without using a helper function. here is how I solved it:
$(window).on('action:topic.loaded', function (event, data) {
var replynum = data.postcount - 1;
console.log(replynum)
});

I have tested it and it works, every time a topic is loaded I can get the postcount.
However, I am still unable to know why my helper function is not being executed

thanks for the reply. I will wait for the major release then

I keep getting this error (more than 25 errors ) when I validate my theme (which is build on the persona theme), will it be an issue if I delete the attribute "component" from my ul, li, h2.....
examples of the errors I get:
Attribute component not allowed on element li at this point.
Attribute component not allowed on element h2 at this point.
Attribute component not allowed on element i at this point.

the answer is in this topic :
https://community.nodebb.org/topic/11906/add-a-class-to-the-body-if-the-template-is-404/3

I tested that and it works great ! thanks again for your help

is there a way to add an if statement to header.tpl where it adds the class "error_page" to the page if the 404 template is rendered (page not found)? this will help me hide the sidebar, for example, using CSS.

@PitaJ thank you so much!

I managed to make some changes on my template from the client side, but am trying to do the same thing on the server side. I am trying to create a helper. but somehow It doesn't work. here what I have done :
first I added the hook on plugin. json :
{ "hook": "filter:topics.get", "method": "clAppendTitle" }
then in library.js I added this code :
var Benchpress = require.main.require('benchpressjs');
.....
.....

library.clAppendTitle = function (params, callback) {
Benchpress.registerHelper('clAppendTitle', function () {
console.log("inside my filter");
});
callback();
};

I am trying to solve this issue for some time now. please someone help ! I am unable to figure out how to create and use helpers. I am using the last version of nodebb.

I have created 404.tpl , and added my custom message. however, I dont want anything else to appear. I tried to use Jquery to do this and it works but after the page is load, so it shows the header and footer for a second then hide them. is it possible to do so before page load ??

thank you !!

ok, I found out how. you simply create a template with the error name inside your theme. example 404.tpl
hopefully, this might help someone.

I am trying to change create a different pattern for the URL. for example :
from this URL : ...../topic/125/topic-title
to this URL : ...../fragen/topic-title

which means I want to replace topic with fragen and remove the topic number. is it possible?