小技巧：
temp1 = document.getElementById('id1');
temp2 = document.getElementById('id2');
which = temp1 || temp2;
如果temp1元素存在，则which指向temp1，否则which指向temp2。
Javascript中每个对象都有一个constructor属性
JavaScript中对象和数组的学习：
以下摘自javacript权威指南
objects and arrays are two fundamental datatypes in JavaScript. They are also two of the most important. Objects and arrays differ from primitive datatypes such as strings and numbers: instead of representing a single value, they are instead collections of values. An object is a collection of named values, and an array is a specialized kind of object that behaves as an ordered collection of numbered values.
(大意)对象和数组是两种基础的数据类型，同时也是比较重要的两种。它们不同与基本的像字符串或数值型数据类型。它们是值的集合。对象是属性的集合，而数组是一种有序集。
You've seen the . operator used to access the properties of an object. It is also possible to use the [] operator, which is more commonly used with arrays, to access these properties. Thus, the following two JavaScript expressions have the same value:
object.propertyobject["property"]可以简单知道，对于对象的属性有两种方式：一种是通过.，一种是类似于数组的方式访问。可以看出，使用第二种方式跟数组完全没有关系。