scan

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

Reduce over time.

💡 You can create Redux-like state management with scan!

Why use scan?

The key distinction of the scan operator when compared to other reduction operators is its continuous accumulation feature. With each emitted value, the accumulator function is applied, and the accumulated result is emitted instantaneously. You can remember this by the phrase "accumulate and emit on-the-go."

The scan operator is highly useful in scenarios that require real-time monitoring and processing, such as tallying scores in a game, where you want to display the updated score each time points are added. However, be cautious when using scan for cases where the only the final accumulated result is crucial. In those situations, the reduce operator may be more appropriate, as it emits only the final value after the source completes.

In summary, the scan operator provides a powerful and flexible means of handling continuous accumulation and emission of values, which can be especially useful in real-time monitoring and processing tasks.

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

Additional Resources

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

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