adder.js

var functions = {
  add: function (args) {
    return args.reduce(function (a, b) { return a + b });
  },
  twoarg: function (a, b) {
    return a * b;
  },
  async: function (a, cb) {
    var state = this.state;
    setTimeout(function () {
      cb(a + state.val);
    }, 100);
  },
  add_state: function (x) {
    this.state.val += x;
  },
  timeout: function (x) {
    setTimeout(function () {
      cb(x);
    }, x);
  },
  call: function (a, b, cb) {
    this.call('multiply', [a, b], function (c) {
      cb(c);
    });
  }
};

function Server(state) {
  this.state = state;
};
// Convert ["fn", 4, 5] to functions.fn.apply(state, [4, 5, cb])
Server.prototype.handle_call = function (args, cb) {
  return functions[args[0]].apply(this, args.slice(1).concat([cb]));
};
Server.prototype.handle_cast = function (args) {
  var cb = function () {};
  return functions[args[0]].apply(this, args.slice(1).concat([cb]));
};

module.exports = Server;

js_gen_server_test.ex

defmodule JSGenServerTest.TestServer do
  use JSGenServer, path: "/fixtures/adder.js"

  def multiply(a, b) do
    a * b
  end
end

defmodule JSGenServerTest do
  use ExUnit.Case

  setup do
    {:ok, pid} = JSGenServerTest.TestServer.start_link(%{val: 10}, name: Adder)
    {:ok, [pid: pid]}
  end

  test "basic functionality", %{pid: pid} do
    assert GenServer.call(pid, {:add, Enum.to_list(1..4)}) == 10
    assert GenServer.call(pid, {:twoarg, 6, 7}) == 42
    assert GenServer.call(pid, {:async, 5}) == 15
  end

  test "named servers" do
    assert GenServer.call(Adder, {:add, Enum.to_list(1..4)}) == 10
  end

  test "external calls" do
    assert GenServer.call(Adder, {:call, 6, 9}) == 54
    assert GenServer.call(Adder, {:call, 7, 9}) == 63
    assert GenServer.call(Adder, {:call, 8, 9}) == 72
  end

  test "casts", %{pid: pid} do
    assert GenServer.call(pid, {:async, 5}) == 15
    assert GenServer.cast(pid, {:add_state, 5})
    assert GenServer.call(pid, {:async, 5}) == 20
  end

  test "timeouts", %{pid: pid} do
    assert {:timeout, {GenServer, :call, [^pid, {:timeout, 500}, 100]}}
           = catch_exit(GenServer.call(pid, {:timeout, 500}, 100))
  end
end