groupBy.js

people = [{name: 'Alice'}, {name: 'Bob'}, {name: 'Bill'}, {name: 'Alex'}, {name: 'John'}];
people.reduce((obj, person) => {
  const firstLetter = person.name[0].toUpperCase();
  return {
    ...obj,
    [firstLetter]: [...(obj[firstLetter] || []), person]
  }
}, {});