scan

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

Reduce over time.

You can create Redux-like state management with scan!

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);

Related Recipes

• Smart Counter
• Progress Bar

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

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