Using bitwise operators in javascript

Question

I am creating a bitmask in javascript. It works fine for bit 0 through 14. When I set only bit fifteen to 1. It yields the integer value of "-2147483648" instead of "2147483648". I can do a special case hack here by returning hardcoded "2147483648" for bit fifteen but I would like to know the correct way of doing it.

Sample code:

function join_bitmap(hex_lower_word, hex_upper_word)
{
    var lower_word = parseInt(hex_lower_word, 16);
    var upper_word = parseInt(hex_upper_word, 16);
    return (0x00000000ffffffff & ((upper_word<<16) | lower_word));
}

Above code returns -2147483648 when hex_lower_word is "0x0" and hex_upper_word is "0x8000" instead of 2147483648

Answer 1

The reason for this is because Javascript's bit shift operations use signed 32-bit integers. So if you do this:

0x1 << 31   // sets the 15th bit of the high word

It will set the sign bit to 1, which means negative.

On the other hand instead of bit shifting you multiply by powers of two, you'll get the result you want:

1 * Math.pow(2, 31)