This is the Euclidean algorithm for GCD, it's naive and simple. But if you see it more carefully, the commit that says "Optimized performance ..." contains just the lines:

  34      var tmp = a;
  35	  a = Math.max(a, b);
  36	  b = Math.min(tmp, b);
  37	  if (a % b === 0) return b;

to avoid the unnecessary repetitions if b is already the GCD