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 )
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// RxJS v6+
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import { from } from 'rxjs';
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import { groupBy, mergeMap, toArray } from 'rxjs/operators';
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​
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const people = [
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  { name: 'Sue', age: 25 },
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  { name: 'Joe', age: 30 },
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  { name: 'Frank', age: 25 },
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  { name: 'Sarah', age: 35 }
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];
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//emit each person
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const source = from(people);
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//group by age
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const example = source.pipe(
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  groupBy(person => person.age),
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  // return each item in group as array
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  mergeMap(group => group.pipe(toArray()))
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);
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/*
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  output:
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  [{age: 25, name: "Sue"},{age: 25, name: "Frank"}]
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  [{age: 30, name: "Joe"}]
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  [{age: 35, name: "Sarah"}]
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*/
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const subscribe = example.subscribe(val => console.log(val));
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Example 2: Group by into key - values
( StackBlitz )
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// RxJS v6+
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import { from, of, zip } from 'rxjs';
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import { groupBy, mergeMap, toArray } from 'rxjs/operators';
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​
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const people = [
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  { name: 'Sue', age: 25 },
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  { name: 'Joe', age: 30 },
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  { name: 'Frank', age: 25 },
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  { name: 'Sarah', age: 35 }
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];
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​
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from(people)
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  .pipe(
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    groupBy(
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      person => person.age,
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      p => p.name
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    ),
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    mergeMap(group => zip(of(group.key), group.pipe(toArray())))
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  )
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  .subscribe(console.log);
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​
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/*
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  output:
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  [25, ["Sue", "Frank"]]
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  [30, ["Joe"]]
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  [35, ["Sarah"]]
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*/
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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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Contents
Group into observables based on provided value.
Examples
Additional Resources