JavaScript % (modulo) gives a negative result for negative numbers

Question

According to Google Calculator (-13) % 64 is 51.

According to Javascript (see this JSBin) it is -13.

How do I fix this

Answer 1

Using Number.prototype is SLOW, because each time you use the prototype method your number is wrapped in an Object. Instead of this:

Number.prototype.mod = function(n) {
  return ((this % n) + n) % n;
}

Use:

function mod(n, m) {
  return ((n % m) + m) % m;
}

See: http://jsperf.com/negative-modulo/2

~97% faster than using prototype. If performance is of importance to you of course..