title: how a node.js add-on check Arguments types
body: <p>I use node.js for a while, now I need write an add-on, but I am a newb of c++.</p>
  
  <p>in node.js I can pass optional arguments to function, and check their types.</p>
  
  <pre><code>function hello(arg, options, callback){
    if(!callback){
      if(typeof options === 'function'){
         callback = options;
         options = {};
      }
    }
    console.log(typeof arg);
  }
  </code></pre>
  
  <p>But in an addon.</p>
  
  <pre><code>Handle&lt;Value&gt; hello(const Arguments &amp;args) {
      HandleScope scope;
      printf("%d\n", args.Length());
      // how to check type of args[i]
      return String::New("world");
  }
  </code></pre>