Modulus (%) : Mod « Operators « JavaScript Tutorial

Modulus (%) operator returns only the remainder.

If either value is a string, an attempt is made to convert the string to a number.

For example, the following line of code

var resultOfMod = 26 % 3;

would result in the remainder of 2 being stored in the variable resultOfMod.

<html>
    <script language="JavaScript">
    <!--
    answer = 7 % 2;
    document.write("answer = ",answer);
    -->
    </script>
</html>

The modulus operator behaves differently for special values:

If the operands are numbers, regular arithmetic division is performed, and the remainder of that division is returned.
If the dividend is Infinity or the divisor is 0, the result is NaN.
If Infinity is divided by Infinity, the result is NaN.
If the divisor is an infinite number, the result is the dividend.
If the dividend is 0, the result is 0.