How to catch the fn key using Hammerspoon

catch_fn.lua

-- Catch fn-h and convert it to a left arrow key.
function catcher(event)
   if event:getFlags()['fn'] and event:getCharacters() == "h" then
      print("fn-h!")
      return true, {hs.eventtap.event.newKeyEvent({}, "left", true)}
   end
   return false
end

local tapper=hs.eventtap.new({hs.eventtap.event.types.keyDown}, catcher):start()