bufferCount

signature: bufferCount(bufferSize: number, startBufferEvery: number = null): Observable

Collect emitted values until provided number is fulfilled, emit as array.

Examples

Example 1: Collect buffer and emit after specified number of values

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { interval } from 'rxjs';
import { bufferCount } from 'rxjs/operators';

//Create an observable that emits a value every second
const source = interval(1000);
//After three values are emitted, pass on as an array of buffered values
const bufferThree = source.pipe(bufferCount(3));
//Print values to console
//ex. output [0,1,2]...[3,4,5]
const subscribe = bufferThree.subscribe(val =>
  console.log('Buffered Values:', val)
);

Example 2: Overlapping buffers

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { interval } from 'rxjs';
import { bufferCount } from 'rxjs/operators';

//Create an observable that emits a value every second
const source = interval(1000);
/*
bufferCount also takes second argument, when to start the next buffer
for instance, if we have a bufferCount of 3 but second argument (startBufferEvery) of 1:
1st interval value:
buffer 1: [0]
2nd interval value:
buffer 1: [0,1]
buffer 2: [1]
3rd interval value:
buffer 1: [0,1,2] Buffer of 3, emit buffer
buffer 2: [1,2]
buffer 3: [2]
4th interval value:
buffer 2: [1,2,3] Buffer of 3, emit buffer
buffer 3: [2, 3]
buffer 4: [3]
*/
const bufferEveryOne = source.pipe(bufferCount(3, 1));
//Print values to console
const subscribe = bufferEveryOne.subscribe(val =>
  console.log('Start Buffer Every 1:', val)
);

Example 3: Last n keyboard presses tracking

( StackBlitz )

// RxJS v6+
import { fromEvent, of } from 'rxjs';
import { bufferCount, map, mergeMap, tap } from 'rxjs/operators';

const fakeKeyPressesPost = keypresses =>
  of(201).pipe(
    tap(_ => {
      console.log(`received key presses are: ${keypresses}`);
      document.getElementById('output').innerText = keypresses;
    })
  );

fromEvent(document, 'keydown')
  .pipe(
    map((e: KeyboardEvent) => e.key),
    bufferCount(5),
    mergeMap(fakeKeyPressesPost)
  )
  .subscribe();

Additional Resources

• bufferCount 📰 - Official docs

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