Non-recursive definition of ackermann function

ackermann.js

// Non-recursive definition
function ack(m,n) {
  var ans, mn, pile;
  for (pile=[[m,n]]; mn = pile.pop();) {
    var m = mn[0], n = mn[1];
    if (n === -1) n = ans;
    if (m === 0) ans = n + 1;
    else if (n === 0) pile.push([m-1, 1]);
    else {
      pile.push([m-1, -1], [m, n-1]);
    }
  }
  return ans;
}

// Recursive definition
function ackr (m, n) {
  if (m===0) return n+1;
  else if (n===0) return ackr(m-1, 1);
  return ackr(m-1, ackr(m, n-1));
}