signature: reduce(accumulator: function, seed: any): Observable

Reduces the values from source observable to a single value that's emitted when the source completes.

💡 Just like Array.prototype.reduce()​

💡 If you need the current accumulated value on each emission, try scan!

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Examples

Example 1: Sum a stream of numbers

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { of } from 'rxjs';
import { reduce } from 'rxjs/operators';
​
const source = of(1, 2, 3, 4);
const example = source.pipe(reduce((acc, val) => acc + val));
//output: Sum: 10'
const subscribe = example.subscribe(val => console.log('Sum:', val));

Additional Resources

• ​reduce 📰 - Official docs

• ​Scan() vs reduce() | RxJS TUTORIAL​

  🎥 - Academind

• ​Build your own reduce operator​

  🎥 - Kwinten Pisman

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