How to detect if a number is even

I always figured this was a nice joke but obviously not code that would ever actually be written by someone... Then I ran into this millimeter to inch conversion code in production this past month:

if (isNaN(mm)) return 0;
if (mm == 6) {
    inch = 0.125;
}
else if (mm == 8) inch = 0.25;
else if (mm == 10) inch = 0.375;
else if (mm == 15) inch = 0.5;
else if (mm == 20) inch = 0.75;
else if (mm == 25) inch = 1;
else if (mm == 30) inch = 1.25;
else if (mm == 40) inch = 1.5;
else if (mm == 50) inch = 2;
else if (mm == 60) inch = 2.5;
else if (mm == 80) inch = 3;
else if (mm == 90) inch = 3.5;
else if (mm == 100) inch = 4;
...