simple-reduce.js

const makeSet = (list) => {
  return list.reduce((items, item) => {
    const id = item.id;
    const value = item.value
    const found = items.find(i => i.id === id);

    if (!found) {
      items.push(item);
    } else {
      found.value += value;
    }

    return items;
  }, []);
};

const list = [
  { id: 1, value: 2 },
  { id: 1, value: 6 },
  { id: 3, value: 1 },
  { id: 3, value: 3 },
  { id: 5, value: 3 },
  { id: 5, value: 10 },
]

const set = makeSet(list);
console.log(set);
/* [ { id: 1, value: 8 }, { id: 3, value: 4 }, { id: 5, value: 13 } ] */