nodemcu-firmware/lua_examples/tcp2uart.lua

do
  uart.setup(0, 9600, 8, 0, 1, 0)
  local sv = net.createServer(net.TCP, 60)
  local global_c = nil
  sv:listen(9999, function(c)
    if global_c~=nil then
      global_c:close()
    end
    global_c = c
    c:on("receive",function(_, pl)	uart.write(0, pl) end)
  end)

  uart.on("data", 4, function(data)
    if global_c ~= nil then
      global_c:send(data)
    end
  end, 0)
end