"Simulate" a 32-bit integer overflow in JavaScript

Question

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?

Answer 1

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.

Answer 2

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;