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JavaScript can handle the following Math just fine:

var result = (20000000 * 48271) % 0x7FFFFFFF;

But in some programming languages, that first int*int multiplication results in a value too large to hold in a standard 32 bit integer. Is there any way to "simulate" this in JavaScript, and see what the resulting calculation would be if the multiplication resulted in an integer overflow?

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  • 1
    possible duplicate of Does Javascript handle integer overflow and underflow? If yes, how? Commented May 10, 2014 at 6:30
  • 1
    @FélixSaparelli It's not a duplicate, 20000000 * 48271 is still well within the JavaScript Number's 52 bit accuracy; it will not overflow. I'm trying to simulate a 32 bit overflow. Commented May 10, 2014 at 6:33
  • I cheated by subtracting 2^32 enough times from the result, but I don't think that's very efficient or very smart :P Commented May 10, 2014 at 6:34
  • if Math.abs(int * int) is greater than (2^32)/2 then log value and continue? Commented May 10, 2014 at 6:45

3 Answers 3

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In newer browsers, Math.imul(a,b) will give you an actual 32-bit integer multiplied result, with overflow resulting the way you would expect (it gives the lower half of the 64-bit result as what it returns).

However, as far as I know there's no way to actually get the overflow, (the upper 32 bits) but the modulus you showed in your answer gets rid of that information, so I figure that's not what you want. If they were going to do overflow, they'd have to separate it based on signed and unsigned anyway.

I know this works in Chrome, Firefox, and Opera, not sure about the rest, though pretty sure IE doesn't have it (typical). You'd need to fall back to a shim such as this one.

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4 Comments

I think you're confusing 32-bit/64-bit with unsigned/signed.
No, when you multiply two 32 bit numbers in two's complement, it produces a 64 bit result, though the lower half will be correct regardless of whether the multiplication is signed or not- it's the upper half, the overflow, that varies based on signedness. See stackoverflow.com/questions/14063599/…
@TND, I'm writing a hashCode like function where the algorithm I've been told to implement has a 32-bit int and expects the behavior provided by the Math.imul(). I would suggest that you remove the part about it having no way to get the upper 32 bits to make the answer clearer.
Is there also iadd? Addition with correct overflow? I need this for my Javascript based JVM
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It is possible to simulate 32-bit integer by "abusing" the bitwise operators available in JavaScript (since they can only return integers within that range).

To convert to a signed 32-bit integer:

x = (a * b) | 0;

To convert to an unsigned 32-bit integer:

x = (a * b) >>> 0;
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2 Comments

The source of the code is from an different question: StackOverflow: JavaScript Integer math incorrect results
This does not handle overflow correctly. Math.imul must be used instead.
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Another way to achieve this is to convert to a format where you can remove extra bytes before converting back to integer. That may not be optimal for JS but can be helpful in environments with really restricted operators.

For instance with hexadecimal:

var hugeInteger = 999999999999999;
var ui32 = parseInt(hugeInteger.toString(16).slice(-8), 16);
// ui32 == 2764472319

It's also possible to get the overflow from hugeInteger.toString(16).slice(0,-8)

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