function XBTNotification(){
    let previousPrice = null;
    let intervalId = null;

    function getCurrentPrice() {
        const currentPriceSpan = document.querySelector('span.priceWidget') || {};
        return parseFloat(currentPriceSpan.textContent || 0);
    }

    function priceComparatorPercent(previousPrice, currentPrice) {
        return (((currentPrice * 100) / previousPrice) - 100) * -1;
    }

    function run(time){
        return Notification.requestPermission()
        .then(permission => {
            if(permission !== 'granted'){
                return alert('Sem permissão não dá pra notificar...');
            }
            previousPrice = getCurrentPrice(); // for first call...
            intervalId = setInterval(() => {
                const currentPrice = getCurrentPrice();
                const percentageVariation = priceComparatorPercent(previousPrice, currentPrice);
                new Notification(`Variação do XBT: ${percentageVariation}`);
                previousPrice = currentPrice; // after used current turn previous.
            }, time);
        })
    }

    function stop(){
        clearInterval(intervalId);
    }

    return {
        run,
        stop
    }
}