scan - Learn RxJS

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scan
signature: scan(accumulator: function, seed: any): Observable
Reduce over time.
💡 You can create Redux-like state management with scan!
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Examples
Example 1: Sum over time
( StackBlitz )
// RxJS v6+
import { of } from 'rxjs';
import { scan } from 'rxjs/operators';
​
const source = of(1, 2, 3);
// basic scan example, sum over time starting with zero
const example = source.pipe(scan((acc, curr) => acc + curr, 0));
// log accumulated values
// output: 1,3,6
const subscribe = example.subscribe(val => console.log(val));
Example 2: Accumulating an object
( StackBlitz | jsBin | jsFiddle )
// RxJS v6+
import { Subject } from 'rxjs';
import { scan } from 'rxjs/operators';
​
const subject = new Subject();
//scan example building an object over time
const example = subject.pipe(
  scan((acc, curr) => Object.assign({}, acc, curr), {})
);
//log accumulated values
const subscribe = example.subscribe(val =>
  console.log('Accumulated object:', val)
);
//next values into subject, adding properties to object
// {name: 'Joe'}
subject.next({ name: 'Joe' });
// {name: 'Joe', age: 30}
subject.next({ age: 30 });
// {name: 'Joe', age: 30, favoriteLanguage: 'JavaScript'}
subject.next({ favoriteLanguage: 'JavaScript' });
Example 3: Emitting random values from the accumulated array.
( StackBlitz )
// RxJS v6+
import { interval } from 'rxjs';
import { scan, map, distinctUntilChanged } from 'rxjs/operators';
​
// Accumulate values in an array, emit random values from this array.
const scanObs = interval(1000)
  .pipe(
    scan((a, c) => [...a, c], []),
    map(r => r[Math.floor(Math.random() * r.length)]),
    distinctUntilChanged()
  )
  .subscribe(console.log);
Example 4: Accumulating http responses over time
( StackBlitz )
// RxJS v6+
import { interval, of } from 'rxjs';
import { scan, delay, repeat, mergeMap } from 'rxjs/operators';
​
const fakeRequest = of('response').pipe(delay(2000));
​
// output:
// ['response'],
// ['response','response'],
// ['response','response','response'],
// etc...
​
interval(1000)
  .pipe(
    mergeMap(_ => fakeRequest),
    scan < string > ((all, current) => [...all, current], [])
  )
  .subscribe(console.log);
Related Recipes
​Alphabet Invasion Game​
​Battleship Game​
​Breakout Game​
​Car Racing Game​
​Catch The Dot Game​
​Click Ninja Game​
​Flappy Bird Game​
​Matrix Digital Rain​
​Memory Game​
​Platform Jumper Game​
​Progress Bar​
​Smart Counter​
​Space Invaders Game​
​Stop Watch​
​Tank Battle Game​
​Tetris Game​
​Uncover Image Game​
Additional Resources
​scan 📰 - Official docs
​Aggregating streams with reduce and scan using RxJS​
🎥 - Ben Lesh
​Updating data with scan​
🎥 💵 - John Linquist
​Transformation operator: scan​
🎥 💵 - André Staltz
​Build your own scan operator​
🎥 - Kwinten Pisman
📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/scan.ts​
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Reduce over time.
Examples
Related Recipes
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