Tail recursive inner functions to remove accumulator vars from the api

test.js

// Non tail recursive; will have a stack overflow.
function sum(list) {
  if(list.length === 0) {
    return 0;
  }

  return list[0] + sum(list.slice(1));
}

// Naive tail recursive (ugly api)
function sum2(list, total = 0) {
  if(list.length === 0) {
    return total;
  }

  return sum2(list.slice(1), total + list[0]);
}

// Tail recursive using an inner function for a clean api
function sum3(list) {
  let sumAccumulator = (list, total) => {
    return list.length === 0 ? 
      total : 
      sumAccumulator(list.slice(1), total + list[0]);
  };

  return sumAccumulator(list, 0);
}