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 會把原型鏈上繼承的屬性也遍歷出來闺属，有其他方法可以遍歷独泞，比如最簡單的下標遍歷和Array的forEach方法
for (var i=0; i < arr.length; i++) {
console.log(i)
}
