Difficulties with implicit syntax #3

index.html

<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/4.1.0/rx.all.min.js"></script>

script.js

var energyUse = new Rx.ReplaySubject(1);
var energyAccumulate = energyUse.scan((acc, value) => acc+value, 0);
energyAccumulate.subscribe(c => console.log(c));

energyUse.onNext(3);
// prints 3
energyUse.onNext(5);
// prints 8
energyUse.onNext(34);
// prints 42