let numbers = [1,2,3,4];

function mean(numbers) {

  let total = 0,i;

  for (i = 0; i < numbers.length; i += 1) {
      total += numbers[i];
  }
  return total / numbers.length;
}

function sum(collection) {
  return collection.reduce((sum, x) => sum + x, 0)
}

List([ 1,2,3,4 ])
  .filter(x => x % 2 === 0)
  .update(sum)