How to emit every n-th value?

Question

I'm using mousemove event to create an observable.

Observable.fromEvent(document, 'mousemove')

I need to emit every 10-th event. What do I do?

Answer 1

I can think of four different ways to do it:

bufferCount()

Observable.range(1, 55)
  .bufferCount(10)
  .map(arr => arr[arr.length - 1])
  .subscribe(val => console.log(val));

windowCount()

Observable.range(1, 55)
  .windowCount(10)
  .switchMap(window => window.takeLast(1))
  .subscribe(val => console.log(val));

debounce()

let source = Observable.range(1, 55).publish();

source
  .debounce(val => debounceNotifier)
  .subscribe(val => console.log(val));

let debounceNotifier = source
  .bufferCount(10)
  .publish();
debounceNotifier.connect();

source.connect();

scan()

Observable.range(1, 55)
  .scan((acc, val) => {
    if (acc.length === 10) {
      acc = [];
    }
    acc.push(val);
    return acc;
  }, [])
  .filter(acc => acc.length === 10)
  .map(acc => acc[acc.length - 1])
  .subscribe(val => console.log(val));

However, when using scan() it'll will discard the last value 55.

See demo for all of them: https://jsbin.com/yagayot/14/edit?js,console

Answer 2

Here's an even simpler and considerably faster approach, which I tested with RxJS 6:

range(1, 10000000)
  .pipe(
    filter(function(value, index) { 
      return index % 10 === 0; 
    }),
  );

This code is twice as fast as the bufferCount and windowCount approaches in the other answer: https://jsperf.com/observable-nth/1

This is likely because the filter operator uses a simple counter, instead of having to keep a buffer of the last n elements. I assume this is even faster when n is larger or the elements themselves are bigger. With RxJS 6, you can also easily make this into your own custom operator:

const takeEveryNth = (n: number) => filter((value, index) => index % n === 0);
// usage: rxjs.range(1, 10000000).pipe(takeEveryNth(10));

It is also the code used in the official docs explaining how to create custom operators: https://github.com/ReactiveX/rxjs/blob/6.2.2/doc/pipeable-operators.md