var obj = {};var obj2 = [];var obj3;

<script>
var obj = {};
console.log(obj);

var obj2 = [];
console.log(obj2);

var obj3;
console.log(obj3);
</script>

打印如下：

Object

Array[0]

undefined

原文：http://www.cnblogs.com/jackiehe/p/4338041.html