reduce

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!

Examples

Example 1: Sum a stream of numbers

( StackBlitz |
jsBin |
jsFiddle )

import { of } from 'rxjs/observable/of';
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

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