JavaScript types

JavaScript has dynamic types. This means that the same variable can be used as different types:
Example
var x; // Now x is undefined
var x = 5; // Now x is a Number
var x = "John"; // Now x is a String

In JavaScript null is "nothing". It is supposed to be something that doesn't exist. Unfortunately, in JavaScript, the data type of null is an object.
You can consider it a bug in JavaScript that typeof null is an object. It should be null.

in JavaScript, an array is actually a special kind of object.

http://www.w3schools.com/js

JavaScript中的数组并不像我们在C或java等语言中遇到的常规数组，在js中数组并不是起始地址+长度构成的一片连续的地址空间。
javascript中数组其实就是个对象,只不过会自动管理一些"数字"属性和length属性罢了。
说的更直接一点,JavaScript中的数组根本没有索引,因为索引应该是数字,而JavaScript中数组的索引其实是字符串。
arr[1]其实就是arr["1"],给arr["1000"] = 1,arr.length也会自动变为1001。
这些表现的根本原因就是：JavaScript中的对象就是字符串到任意值的键值对。
虽然稀疏数组和密集数组差别不大，javascript也没有语法强制数组是稀疏的还是密集的，这不过是概念上的区分。

这行代码等同于var  dense =  Array(undefined, undefined, undefined) 看代码：

//稀疏数组
 var array = new Array(3); 
 array[2] = "name";
 
 for(var a in array) 
 {
    console.log("index=" + a + ",value=" + array[a]);
 }
 
 // 密集数组
 var dense = Array.apply(null, Array(3)); 
 dense[2] = "name";
 for(var a in dense) 
 {
    console.log("index=" + a + ",value=" + dense[a]);
 }

http://www.tuicool.com/articles/AfYnEf