NaN and 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

Both NaN and Infinity are of type "number":

typeof NaN // "number"
typeof Infinity // "number"

Note that NaN compares in a strange way:

NaN == NaN // false (!)

But:

Infinity == Infinity // true