RxJS 4 Observable.prototype.pairwise 的な関数を RxJS 5 で

pairwise-test.ts

import assert from 'power-assert';
import { Observable } from 'rxjs';
import pairwise from '../src/pairwise';

describe('pairwise', function() {
  context('when []', function() {
    it('works', function() {
      const source = Observable.fromArray([]);
      pairwise(source)
        .toArray()
        .subscribe(array => {
          assert(array.length === 0);
        });
    });
  });

  context('when [1]', function() {
    it('works', function() {
      const source = Observable.fromArray([1]);
      pairwise(source)
        .toArray()
        .subscribe(array => {
          assert(array.length === 0);
        });
    });
  });

  context('when [1, 2, 3, 4]', function() {
    it('works', function() {
      const source = Observable.fromArray([1, 2, 3, 4]);
      pairwise(source)
        .toArray()
        .subscribe(array => {
          assert(array.length === 3);
          const [i1, i2, i3] = array;
          assert.deepEqual(i1, [1, 2]);
          assert.deepEqual(i2, [2, 3]);
          assert.deepEqual(i3, [3, 4]);
        });
    });
  });
});

pairwise.ts

import { Observable } from 'rxjs';

export default function pairwise<T>(
  observable: Observable<T>
): Observable<[T, T]> {
  return observable
    .scan<{ hp: boolean; p: T; pair: [T, T] }>(({ hp, p }, x) => {
      return { hp: true, p: x, pair: (hp ? [p, x] : null) };
    }, { hp: false, p: null, pair: null })
    .map(({ pair }) => pair)
    .filter(pair => pair !== null);
}

https://github.com/Reactive-Extensions/RxJS/blob/v4.0.7/doc/api/core/operators/pairwise.md
https://github.com/bouzuya/rxjs-operator-examples/tree/bbc49fc2d9c43c3279960de78f114f59270359dc