bestProfit.js

const prices = [10, 7, 5, 12, 1, 89];

const initialState = {
    maxProfit: 0,
    bestBuyPrice: prices[0],
};

const bestProfit$ = Rx
.Observable
.from(prices)
.scan(({ maxProfit, bestBuyPrice }, currentPrice) => {
    const potentialProfit = currentPrice - bestBuyPrice;
    return {
        maxProfit: Math.max(maxProfit, potentialProfit),
        bestBuyPrice: Math.min(currentPrice, bestBuyPrice);
    }
}, initialState)
.flatMap(obj => Rx.Observable.of(obj))

bestProfit$.subscribe(console.log)