http://stackoverflow.com/questions/500504/why-is-using-for-in-with-array-iteration-a-bad-idea
The purpose of the for-in statement is to enumerate over object properties. This statement will go up in the prototype chain, also enumerating over inherited properties, a thing that sometimes is not desired.
for in 会把原型链上继承的属性也遍历出来，有其他方法可以遍历，比如最简单的下标遍历和Array的forEach方法
for (var i=0; i < arr.length; i++) {
console.log(i)
}
