Question

This JavaScript demonstrates a bug in IE that is driving me nuts right now:

var y = 6044629098073143; // this exact integer easily fits into an IEEE double
document.write(y + " " + Math.round(y)+"<br><br>");

The output in IE 8 (and Opera 12.02) shows that Math.round is off by 1:

6044629098073143 6044629098073144

Output in Firefox, Chrome and Safari is correct.

What on earth is going on here with IE and Opera?

Answer 1

I’ve confirmed RobG’s comments: All integers below 2^52 (4503599627370496) appear to round correctly in all browsers. Integers higher than this round UP to an EVEN number in IE/Opera with Math.round (whereas other browsers round correctly).

As RobG mentioned, it is likely that IE and Opera implement Math.round(x) as Math.floor(x + 0.5), which leads to these results since adding 0.5 to a value > 2^52 yields an imprecise result. A smarter implementation of round() would use the FPU’s native rounding support (IE and Opera developers, please take note and fix!)

In the meantime, this workaround should work:

// workaround for IE and Opera's brain-dead Math.round() implementation
if (y < 4503599627370496) // 2^52
{
  y = Math.round(y);
}
// else 'y' is already an integer