// undefined nullの違い
typeof undefined    // undefined
typeof null         // object
null === undefined  // false
null == undefined   // true

// javaScriptよく使うデータ型
typeof "John"                 // string
typeof 3.14                   // number
typeof NaN                    // number
typeof false                  // boolean
typeof [1,2,3,4]              // object
typeof {name:'John', age:34}  // object
typeof new Date()             // object
typeof function () {}         // function
typeof myCar                  // undefined 
typeof null                   // object

// コンストラクタconstructor
例：
<body>
 <p id="test"></p>
 <script>
  var pTest = document.getElementById("test");
  pTest.innerHTML = 
   "john".constructor + "<br>" +
    (3.14).constructor + "<br>" +
    false.constructor + "<br>" +
    [1,2,3,4].constructor + "<br>" +
    {name:'john', age:34}.constructor + "<br>" +
    new Date().constructor + "<br>" +
    function () {}.constructor;
 </script>
</body>

// 上記<p id="test"></p>内容
function String() { [native code] }
function Number() { [native code] }
function Boolean() { [native code] }
function Array() { [native code] }
function Object() { [native code] }
function Date() { [native code] }
function Function() { [native code] }