14.js

// С рекурсией (медленно)
const calcSum = list => list.reduce((acc, val) => acc + val, 0);

const cumulativeSum = (list, acc = []) => {
  if (!list.length) return acc;
  
  const sum = calcSum(list),
        subset = list.slice(0, list.length - 1);
  
  return cumulativeSum(subset, [sum, ...acc]);
}


// Без (быстро)
const cumulativeSum = list => list.reduce((acc, val) => {
  const last = acc[acc.length-1] || 0;

  return [...acc, val + last];
}, [])