key array of objects by multiple keys

key_by_multiple.js

a = [ { name: 'bill', group: 'b' },
  { name: 'ann', group: 'a' },
  { name: 'ken', group: 'k' },
  { name: 'alex', group: 'a' } ]

_.reduce((sum, v) => {
  const { name, group } = v;
  const current = _.getOr([], [group, name])(sum);
  return _.set([group, name], [...current, v])(sum);
}, {})(a);