Groups the items emitted by an Observable according to a specified criterion, and emits these grouped items as GroupedObservables, one GroupedObservable per group.

When the Observable emits an item, a key is computed for this item with the key function.

If a GroupedObservable for this key exists, this GroupedObservable emits. Otherwise, a new GroupedObservable for this key is created and emits.

A GroupedObservable represents values belonging to the same group represented by a common key. The common key is available as the key field of a GroupedObservable instance.

The elements emitted by GroupedObservables are by default the items emitted by the Observable, or elements returned by the element function.

Examples

Group objects by id and return as array

import { of, groupBy, mergeMap, reduce } from 'rxjs';

of(
  { id: 1, name: 'JavaScript' },
  { id: 2, name: 'Parcel' },
  { id: 2, name: 'webpack' },
  { id: 1, name: 'TypeScript' },
  { id: 3, name: 'TSLint' }
).pipe(
  groupBy(p => p.id),
  mergeMap(group$ => group$.pipe(reduce((acc, cur) => [...acc, cur], [])))
)
.subscribe(p => console.log(p));

// displays:
// [{ id: 1, name: 'JavaScript' }, { id: 1, name: 'TypeScript'}]
// [{ id: 2, name: 'Parcel' }, { id: 2, name: 'webpack'}]
// [{ id: 3, name: 'TSLint' }]

Pivot data on the id field

import { of, groupBy, mergeMap, reduce, map } from 'rxjs';

of(
  { id: 1, name: 'JavaScript' },
  { id: 2, name: 'Parcel' },
  { id: 2, name: 'webpack' },
  { id: 1, name: 'TypeScript' },
  { id: 3, name: 'TSLint' }
).pipe(
  groupBy(p => p.id, { element: p => p.name }),
  mergeMap(group$ => group$.pipe(reduce((acc, cur) => [...acc, cur], [`${ group$.key }`]))),
  map(arr => ({ id: parseInt(arr[0], 10), values: arr.slice(1) }))
)
.subscribe(p => console.log(p));

// displays:
// { id: 1, values: [ 'JavaScript', 'TypeScript' ] }
// { id: 2, values: [ 'Parcel', 'webpack' ] }
// { id: 3, values: [ 'TSLint' ] }