One-line group an Array by an Object Property - JavaScript ES2015, ES6

groupByObjProp.js

const groupBy = (arr, groupFn) => arr.reduce((grouped, obj) => ({...grouped,[groupFn(obj)]: [...(grouped[groupFn(obj)] || []), obj], }), {});

// Using it
const people = [
  { name: 'Matt' },
  { name: 'Sam' },
  { name: 'John' },
  { name: 'Mac' },
];
groupBy(people, (person) => person.name.length);