Group By - ES6

groupBy.js

function groupBy(list, props) {
  return list.reduce((a, b) => {
     (a[b[props]] = a[b[props]] || []).push(b);
     return a;
  }, {});
}

// Usage.
groupBy([{
  id: 1,
  group: 'user',
  name: 'User #1',
}, {
  id: 2,
  group: 'admin',
  name: 'User #2',
}, {
  id: 3,
  group: 'moderator',
  name: 'User #3',
}, {
  id: 4,
  group: 'user',
  name: 'User #4',
}, {
  id: 5,
  group: 'moderator',
  name: 'User #5',
}], 'group');

/*
{admin: [], user: [], moderator: []}
*/