A vanilla scrollTop implementation (includes throttle and scrollTo utility functions)

jump.js

/**
 * Vanilla scrollTo implementation (smoothly scroll/"jump" to an element)
 * @author github.com/andreasvirkus
 *
 * Default easing is:
 *  Robert Penner's easeInOutQuad - http://robertpenner.com/easing/
 *
 * @param {String} target selector to scroll to
 * @param {Object} options Optionally defined duration, offset, callback and easing
 *
 * @return {void}
 */
export default function jump(target, options) {
	target = target === '#' ? (options.offset || 0) : target;

	let start = window.pageYOffset,
		opt = {
			duration: options.duration,
			offset: options.offset || 0,
			callback: options.callback,
			easing: options.easing || easeInOutQuad
		},
		distance = typeof target === 'string'
			? opt.offset + document.querySelector(target).getBoundingClientRect().top
			: target,
		duration = typeof opt.duration === 'function'
			? opt.duration(distance)
			: opt.duration,
		timeStart, timeElapsed;

	requestAnimationFrame(function(time) {
		timeStart = time;
		loop(time);
	});

	function loop(time) {
		timeElapsed = time - timeStart;

		window.scrollTo(0, opt.easing(timeElapsed, start, distance, duration));

		console.log('scrolling...', start, timeElapsed);

		if (timeElapsed < duration)
			requestAnimationFrame(loop);
		else
			end();
	}

	function end() {
		window.scrollTo(0, start + parseInt(distance, 10));

		if (typeof opt.callback === 'function')
			opt.callback();
	}

	function easeInOutQuad(t, b, c, d)  {
		t /= d / 2;
		if (t < 1) return c / 2 * t * t + b;
		t--;

		return -c / 2 * (t * (t - 2) - 1) + b;
	}
}

throttle.js

// Used in the scrollToTop example
function throttle(fn, wait) {
    var time = Date.now();

    return function() {
        if (time + wait - Date.now() < 0) {
            fn();
            time = Date.now();
        }
    };
}

usage.js

/* global throttle jump */
// A simple scrollToTop implementation
(function() {
    var toTop = document.getElementById('back-to-top'),
        displayOffset = 250,
        offset;

    showHideTopButton();
    window.addEventListener('scroll', throttle(showHideTopButton, 250));

    toTop.onclick = function () {
        jump('main', {
            duration: 200,
            callback: function () {
                toTop.classList.remove('active');
            }
        });
    };

    /**
     * Decide whether to show or hide the scrollToTop button
     * depending on how far the viewport has been scrolled
     *
     * @return {void}
     */
    function showHideTopButton() {
        offset = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop || 0;

        if (offset > displayOffset) {
            toTop.classList.add('active');
        } else {
            toTop.classList.remove('active');
        }
    }
}());