port_server.ex

defmodule App.Thing do
  use GenServer

  def init(%{code: code}) do
    Process.flag(:trap_exit, true)
    handler = self()

    {:ok, task} = Task.start_link(fn ->
      port = Port.open(
        {:spawn_executable, System.find_executable("npm")},
        [:binary, :use_stdio, :stderr_to_stdout, :exit_status,
         {:args, ['-s', 'run', 'server']},
         {:packet, 4},
         {:cd, Path.absname('./node/')}]
      )

      send(handler, {:port, port})

      receive_loop(handler)
    end)

    port = receive do
      {:port, new_port} -> new_port
    after
      500 -> raise "Port never spawned anything"
    end

    {:ok, %{port: port, task: task}}
  end

  def receive_loop(handler) do
    receive do
      {_, {:data, data}} ->
        term = :erlang.binary_to_term(data, [:safe])
        send(handler, {:data, term})
        receive_loop(handler)
      after
        10_000 ->
          receive_loop(handler)
    end
  end
  
  def handle_info({:data, data}, state) do
    handle_port_data(data, state)
  end
end