# 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](/learn-rxjs/operators/transformation/scan.md)!

### 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

***

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


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