js在计算数值时有2个值比较特殊，分别是：NaN 和 Infinity (-Infinity)

Parsing something that isn't a number results in NaN. isNaN helps to detect those cases:

parseInt("hello", 10) // NaN
isNaN(parseInt("hello", 10)) // true

Division through zero results in Infinity:

1 / 0 // Infinity