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

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

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

How do I fix this?

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

Taken from this article: The JavaScript Modulo Bug

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..

The % operator in JavaScript is the remainder operator, not the modulo operator (the main difference being in how negative numbers are treated):

-1 % 8 // -1, not 7

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