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

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

Taken from this article: The JavaScript Modulo Bug

Answer 2

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..