unkeyBy (inverse of _.keyBy)

unkeyBy.js

// using map - fastest!
const unkeyBy = (obj, prop) => {
  const res = [];
  Object.keys(obj).map((x, i) => res[i] = { ...obj[x], [prop]: x });
  return res;
};

// adds ungrouping
const ungroupBy = (obj, prop) => {
  const res = [];
  Object.keys(obj).map(k => {
    const addKey = item => res.push({ ...item, [prop]: k });
    Array.isArray(obj[k]) ? obj[k].map(addKey) : addKey(obj[k]);
  });
  return res;
};

// using reduce and destructuring
const unkeyBy = (obj, prop) => {
  return Object.keys(obj).reduce((memo, key) => {
    return [ ...memo, { ...obj[key], [prop]: key } ];
  }, []);
};

// using reduce and concat
const unkeyBy = (obj, prop) => {
  return Object.keys(obj).reduce((memo, key) => {
    return memo.concat({ ...obj[key], [prop]: key });
  }, []);
};