problem with bitmask and javascript

Question

I am not sure if I am doing something wrong, but I can't seem to have this simple javascript to work:

var a = 0;

a |= (1 << 31);

alert(a);

a |= (1 << 30);

alert(a);

you can see it here http://jsfiddle.net/qPEVk/

shoudln't it 3221225472 ?

thanks,
Joe

user578895 · Accepted Answer · 2011-04-18 16:40:32Z

4

There is technically nothing wrong with that, and a negative number is expected because it's casting to a 32bit signed int.

Basically, the leading bit means "negative or positive", so when you flip it (with 1<<31) you get a negative number.

Your bitmask will still work exactly like you expect on up to 32 bits. You can't exceed a 32-bit bitmask in JavaScript.

answered Apr 18, 2011 at 16:40

user578895

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4 Comments

El Guapo Over a year ago

@Johnathan... check out this post... stackoverflow.com/questions/1908492/…

Jonathan Over a year ago

but the second alert gives me a weird number, should I cast it to positive to send it via post?

user578895 Over a year ago

@Jonathan - No, if you cast it to positive you mess up a lot of things. The 2nd alert is simply 11000000 00000000 00000000 00000000 as a SIGNED int. Your post will understand it properly in just about any language. If you're really concerned about it (or just seems too weird), restrict yourself to 31 bits.

Jonathan Over a year ago

thanks, I need to use the 32 flags, will keep it that way as you suggested, ty!

Mark Costello · Accepted Answer · 2011-04-18 16:43:02Z

0

var a = 0;
var b;
a |= (1 << 31);
b = a
a |= (1 << 30);
b += a
alert(b);

In the above case, b will end up as -3221225472.

answered Apr 18, 2011 at 16:43

Mark Costello

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