Bookmarklet для отображения таймера в верхней части страницы

timer.js

javascript: (function() {
	let interval;
	let containerElt;
	let minElt;
	let secElt;
	const template =
		'<div id="aalexeev-timer" style="background: #434343; padding: 10px; text-align: center; font-size: 40px; font-family: monospace; color: #fff;"><span data-minutes></span>:<span data-seconds></span></div>';
    	const DEFAULT_MINUTES = 10;
    
    	starter();
    
	function starter() {
		var minutes = prompt('На сколько минут?', DEFAULT_MINUTES);

		if (minutes) {
		    countdown(minutes);   
		}
	}
	function countdown(minutes) {
		if (document.getElementById('aalexeev-timer')) {
			return;
		}
		const container = document.createElement('div');
		container.innerHTML = template;
		containerElt = container.firstChild;
		document.body.insertBefore(containerElt, document.body.firstChild);
		minElt = containerElt.querySelector('[data-minutes]');
		secElt = containerElt.querySelector('[data-seconds]');
		endDate = new Date(new Date().getTime() + minutes * 60 * 1000);
		if (isNaN(endDate) || !minElt || !secElt) {
			return;
		}
		calculate(endDate);
		interval = setInterval(() => calculate(endDate), 500);
	}
	function calculate(endDateTime) {
		let startDate = new Date();
		startDate = startDate.getTime();
		let timeRemaining = endDateTime - startDate;
		if (timeRemaining >= 0) {
			renderRemaining(timeRemaining);
		} else {
			clearInterval(interval);
			containerElt.style.color = '#f44336';
		}
	}
	function renderRemaining(timeRemainingMS) {
		const timeRemainingSec = parseInt(timeRemainingMS / 1000, 10);
		const seconds = parseInt(timeRemainingSec % 60).toString();
		let minutes = parseInt(timeRemainingSec / 60, 10).toString();
		if (minutes.length < 2) {
			minutes = '0' + minutes;
		}
		minElt.innerHTML = minutes;
		secElt.innerHTML = ('0' + seconds).slice(-2);
	}
})();