-- following best practice by putting local
-- functions first before main()

local function addZeros(Id, length)
   Id = string.rep('0', length-#Id)..Id
   return Id
end

local function removeZeros(Id)
   while true do 
      if Id:sub(1,1) == '0' then 
         Id = Id:sub(2)
      else
         break
      end
   end   
   return Id
end

function main()
   local Id1 = removeZeros('0004589787')
   local Id2 = addZeros('4589787', 10)
   trace(Id1)
   trace(Id2)
end