groupBy

signature: groupBy(keySelector: Function, elementSelector: Function): Observable

Group into observables based on provided value.

Examples

Example 1: Group by property

( StackBlitz | jsBin | jsFiddle )

// 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));

Example 2: Group by into key - values

( StackBlitz )

// 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"]]
*/

Additional Resources

• groupBy 📰 - Official docs

• Group higher order observables with RxJS groupBy 🎥 💵 - André Staltz

• Use groupBy in real RxJS applications 🎥 💵 - André Staltz

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📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/groupBy.ts