Array.prototype.foldLeft = function (sum, callback) {
  var head,
      list = Array.prototype.slice.call(this);

  if (list.length) {
    head = list.shift(1);
    return list.foldLeft(callback(sum, head), callback);
  }
  return sum;
};

var numbers = [1, 2, 3, 4];
var sum = numbers.foldLeft(0, function (total, next) {
  return total + next;
});
var times = numbers.foldLeft(1, function (total, next) {
  return total * next;
});
console.log('sum ' + sum);
console.log('times ' + times);