binding.gyp

{
  'targets': [
    {
      'target_name': 'math',
      'sources': [
        'factorial.cc'
      ]
    }
  ]
}

factorial.cc

#include <node.h>

double factorial(double n) {
  if (n == 0)
    return 1;
  return n * factorial(n - 1);
}

void factorial(const v8::FunctionCallbackInfo<v8::Value>& info) {
  info
    .GetReturnValue()
    .Set(v8::Number::New(
      info.GetIsolate(),
      factorial(info[0]->NumberValue())
    ));
}

void Init(v8::Local<v8::Object> exports) {
  NODE_SET_METHOD(exports, "factorial", factorial);
}

NODE_MODULE(addon, Init)

factorial.js

(function () {
  'use strict';
  
  var addon = require('./build/Release/factorial.node');
  
  function factorial(n) {
    if (n === 0)
      return 1;
    return n * factorial(n - 1);
  }
  
  console.log(addon.factorial(10), 'should equal', factorial(10));
}());