// RxJS v6+
import { from } from 'rxjs';
import { groupBy, mergeMap, toArray } from 'rxjs/operators';
​
const people = [
  { name: 'Sue', age: 25 },
  { name: 'Joe', age: 30 },
  { name: 'Frank', age: 25 },
  { name: 'Sarah', age: 35 }
];
//emit each person
const source = from(people);
//group by age
const example = source.pipe(
  groupBy(person => person.age),
  // return each item in group as array
  mergeMap(group => group.pipe(toArray()))
);
/*
  output:
  [{age: 25, name: "Sue"},{age: 25, name: "Frank"}]
  [{age: 30, name: "Joe"}]
  [{age: 35, name: "Sarah"}]
*/
const subscribe = example.subscribe(val => console.log(val));

// RxJS v6+
import { from, of, zip } from 'rxjs';
import { groupBy, mergeMap, toArray } from 'rxjs/operators';
​
const people = [
  { name: 'Sue', age: 25 },
  { name: 'Joe', age: 30 },
  { name: 'Frank', age: 25 },
  { name: 'Sarah', age: 35 }
];
​
from(people)
  .pipe(
    groupBy(person => person.age, p => p.name),
    mergeMap(group => zip(of(group.key), group.pipe(toArray())))
  )
  .subscribe(console.log);
​
/*
  output:
  [25, ["Sue", "Frank"]]
  [30, ["Joe"]]
  [35, ["Sarah"]]
*/