books-grokking_alghorithms/04/sum.js

const sum = (arr) => {
  if (arr.length < 2) {
    return arr[0] || 0;
  }

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

const res = sum([1, 2, 3, 4, 5, 6, 8])
console.log('res', res);