変数.constructor

<input type="button" value="ボタン" onclick="a();" /><br /><br />
<script type="text/javascript">
<!--
function a(){
    var b = 3;
    var c = new String("aaa");
    var d = new Date();
    alert(b.constructor);
    alert(c.constructor);
    alert(d.constructor);
}
-->
</script>

<input type="button" value="ボタン" onclick="b();" /><br /><br />
<script type="text/javascript">
<!--
function b(){
    var b = 3;
    if(b.constructor === Number){
        alert(b.constructor);
    }
    if(b.constructor == Number){
        alert(b.constructor);
    }
}
-->
</script>