Massive thank youuuuu!!! the show_modal function was already aliased!
The reason for that is: I looked at lib/your_app_web/your_app_web.ex,
unquote(html_helpers())← this
defp html_helpers do
quote do
# HTML escaping functionality
import Phoenix.HTML
# Core UI components and translation
import HangmanWeb.CoreComponents
import HangmanWeb.Gettext
# Shortcut for generating JS commands
alias Phoenix.LiveView.JS
# Routes generation with the ~p sigil
unquote(verified_routes())
end
end
Took me lots of hours why JS.show_modal is undefined on the page 
