Example how we can sort out array of users by age with multiple emissions process them and change to appropriate structure and from multiple emissions to a single using combineAll()

sort-by-using-rxjs.ts

import { from, of, zip } from 'rxjs';
import { combineAll, groupBy, map, mergeMap, toArray } from 'rxjs/operators';

const people = [
  { id: 1, name: 'Sue', age: 25 },
  { id: 2, name: 'Joe', age: 30 },
  { id: 3, name: 'Frank', age: 25 },
  { id: 4, name: 'Sarah', age: 35 }
];

from(people)
  .pipe(
    groupBy(person => person.age),
    mergeMap(group => zip(of(group.key), group.pipe(toArray()))),
    map(([age, users]) => of({ age, users })),
    combineAll()
  )
  .subscribe((data) => console.log(JSON.stringify(data, null, 2)));

/*
[
  {
    "age": 25,
    "users": [
      {
        "name": "Sue",
        "age": 25
      },
      {
        "name": "Frank",
        "age": 25
      }
    ]
  },
  {
    "age": 30,
    "users": [
      {
        "name": "Joe",
        "age": 30
      }
    ]
  },
  {
    "age": 35,
    "users": [
      {
        "name": "Sarah",
        "age": 35
      }
    ]
  }
]
*/