javascriptbit-manipulationunsigned

Unsigned Integer in Javascript

I'm working on a page that processes IP address information, but it's choking on the fact that integers are signed. I am using bitwise operators to speed it up, but the 64th bit (signed/unsigned flag) is messing it up.

Is there any way to force a number to be unsigned in Javascript? It seems to work fine, until subnet is greater than 30, or less than 2.

Try this:

<html>
    <body>
    
    <script type='text/javascript'>
    document.write( (1 << 30) +"<br/>");
    document.write( (1 << 31) +"<br/>");
    document.write( (1 << 32) +"<br/>");
    </script>
    
    </body>
</html>

Result:

1073741824 -2147483648 1

Solution 
  • document.write( (1 << 31) +"<br/>");

    The << operator is defined as working on signed 32-bit integers (converted from the native Number storage of double-precision float). So 1<<31 must result in a negative number.

    The only JavaScript operator that works using unsigned 32-bit integers is >>>. You can exploit this to convert a signed-integer-in-Number you've been working on with the other bitwise operators to an unsigned-integer-in-Number:

    document.write(( (1<<31)>>>0 )+'<br />');

    Meanwhile:

    document.write( (1 << 32) +"<br/>");

    won't work because all shift operations use only the lowest 5 bits of shift (in JavaScript and other C-like languages too). <<32 is equal to <<0, ie. no change.