javascript странное поведение.

знаешь что такое ++?
вот тебе и NaN!

Это объясняется приоритетом операторов

1. + string is NaN 2. string + NaN -> string NaN

Приведением типов же.

alert(+"1"); // 1
alert(+"str"); //Nan

Кстати, ++ тут не при чем.

The unary plus operator precedes its operand and evaluates to its operand but attempts to converts it into a number, if it isn't already. Although unary negation (-) also can convert non-numbers, unary plus is the fastest and preferred way of converting something into a number, because it does not perform any other operations on the number. It can convert string representations of integers and floats, as well as the non-string values true, false, and null. Integers in both decimal and hexadecimal («0x»-prefixed) formats are supported. Negative numbers are supported (though not for hex). If it cannot parse a particular value, it will evaluate to NaN.

Кстати, ++ тут не при чем

Понятно, восклицательные знаки просто привелись к числу NaN с помощью унарного плюса.