> For the complete documentation index, see [llms.txt](https://chasethestar.gitbook.io/ko.learn-rxjs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://chasethestar.gitbook.io/ko.learn-rxjs/rxjs/operators/transformation/reduce.md).

# 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()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce?v=a)

💡 If you need the current accumulated value on each emission, try [scan](/ko.learn-rxjs/rxjs/operators/transformation/scan.md)!

[![Ultimate RxJS](https://drive.google.com/uc?export=view\&id=1qq2-q-eVe-F_-d0eSvTyqaGRjpfLDdJz)](https://ultimatecourses.com/courses/rxjs?ref=4)

### Examples

**Example 1: Sum a stream of numbers**

( [StackBlitz](https://stackblitz.com/edit/typescript-hdsv5e?file=index.ts\&devtoolsheight=100) | [jsBin](http://jsbin.com/dakuneneho/edit?js,console) | [jsFiddle](https://jsfiddle.net/f8fw7yka/) )

```js
// 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](https://rxjs.dev/api/operators/reduce) 📰 - Official docs
* [Scan() vs reduce() | RxJS TUTORIAL](https://www.youtube.com/watch?v=myEeo2rZc3g) 🎥 - Academind
* [Build your own reduce operator](https://blog.strongbrew.io/build-the-operators-from-rxjs-from-scratch/?lectureId=reduce#app) 🎥 - Kwinten Pisman

***

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