Buffer max count operator for RxJS.

buffer-max-count.js

import { defer } from 'rxjs';
import { map } from 'rxjs/operators';

export const bufferMaxCount = (count = 0) => source => defer(() => {
  let buffer = [];

  return source.pipe(
    map(next => buffer = [next, ...buffer.slice(0, count - 1)])
  );
});