How To Create A 404 Page In HTML/CSS/JS

I want to create a 404 page for my website I made in repl.it, which is a web-editor.

It can either redirect to a page like website.repl.co/404 when the URL is not valid or directly show something representing 404.

The problem is, well, I don't know how to do something when there is such a situation.

I guess using javascript and an extra module to control it?

if(!validate(page.link)){ //checking if validating resulted false
  //redirect to the 404 page (location.replace('/404.html') works i guess)

or

if(!validate(page.link)){ //checking if validating resulted false again
  //show some html here