Немного короче:
const format = ({ items }) => ({
items: items.map((el, id) => ({ id, title: el.title, link: el.link, score: el.score, tags: el.tags }))
});
Или используя некоторые вспомогательные функции:
const lens = (key, fn) => obj => ({ ...obj, [key]: fn(obj[key]) });
const pick = (...keys) => obj => Object.assign(...keys.map(k => ({ [k]: obj[k] })));
const map = (...fns) => arr => arr.map((el, i) => fns.reduce((el, fn) => fn(el, i), el));
const format = lens("items",
map(
pick("title", "link", "score", "tags"),
(el, index) => el.id = index
)
);