Unsigned Integer in Javascript

49
votes

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

7
javascriptbit-manipulationunsigned
You should probably process the different parts of the IP address as several numbers, to avoid overflow issues. – Eric Bréchemier
1<<32 results in a 33-bit number. That wouldn't fit in a C/C++ unsigned int either. The other two results are correct. – moonshadow
@Ates Goral - It is simple when compared to C++. I'm mainly speaking about the fact that types do not exist in JS. Maybe instead of "simple", I mean "less powerful". – bradlis7
I think that using individual bits is the wrong way to go. It's seldom justifiable in C (except for enums), and I'm quite sure never in Javascript or other higher level languages. – Georg Schölly
I'm hosting this program at tools.bradlis7.com/ip for now. – bradlis7

7 Answers

90
votes
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.

10
votes

Douglas Crockford believes that bitwise operators is one of the bad parts of javascript:

In Java, the bitwise operators work with integers. JavaScript doesn't have integers. It only has double precision floating-point numbers. So, the bitwise operators convert their number operands into integers, do their business, and then convert them back. In most languages, these operators are very close to the hardware and very fast. In JavaScript, they are very far from the hardware and very slow. JavaScript is rarely used for doing bit manipulation.

-- Douglas Crockford in "JavaScript: The Good Parts", Appendix B, Bitwise Operators (emphasis added)

Are you sure that bitwise operators really speed up your logic?

9
votes

Use >>> instead of >> to get an unsigned right shift instead of a sign-extending one. All the other bitwise operators behave the same way regardless of whether ints are signed or not.

Your code breaking "when subnet ... is less than 2" is concerning. That sounds like you may have some bug unrelated to signedness of integers.

6
votes

Javascript doesn't have integers, all numbers are actually doubles.

The Javascript 1.5 Reference by Mozilla suggests that one can only use bitwise-operations safely for 32 bit numbers.

4
votes

Here are two functions that convert ipv4 addresses to/from unsigned integers in javascript:

function ip2long(ip) {
    var ipl=0;
    ip.split('.').forEach(function( octet ) {
        ipl<<=8;
        ipl+=parseInt(octet);
    });
    return(ipl >>>0);
}

function long2ip (ipl) {
    return ( (ipl>>>24) +'.' +
        (ipl>>16 & 255) +'.' +
        (ipl>>8 & 255) +'.' +
        (ipl & 255) );
}
0
votes

What kind of IP addresses do you have? IPv4 uses only 32bit addresses, so JavaScript should be fine (using double which gives you an 52bit integer part). IPv6 uses 128bit addresses, so you'll have to use an array. My guess is that something else is broken.

[EDIT] Build a small library which uses an array of two ints as the internal data type.

0
votes

Javascript now has a bigint type, you can use the n suffix to make such a number.

So using your example you could do:

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