groupBy

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groupBy
signature: groupBy(keySelector: Function, elementSelector: Function): Observable
Group into observables based on provided value.
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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
📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/groupBy.ts​
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Last modified 2yr ago
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Outline
Group into observables based on provided value.
Examples
Additional Resources