function isNumber(n) {
  return getType(n) === NUMBER && !isNaN(n)
}
Check if value is number.
@param n
@return {boolean}
@example
> isNumber(0)
true
> isNumber(1)
true
> isNumber(1.1)
true
> isNumber(0xff)
true
> isNumber(0644)
true
> isNumber(6.2e5)
true
> isNumber(NaN)
false
> isNumber(Infinity)
true