List Files in Lua on NodeMCU ESP8266

l = file.list()
str = ""
if l == nil then
  str = "{\"files\":null}"
else
  str = "{\"files\":["
  for k,v in pairs(l) do
    str = str .. "{\"name\":\"" .. k .. "\", \"size\":" .. v .. "}, "
  end
  str = string.sub(str, 0, string.len(str) - 2)
  str = str .. "]}"
end
print(str)
str = nil
l = nil