js parallax inertia

index.html

<div class="js-parallax" data-parallax="-0.05"></div>
<div class="js-parallax" data-parallax="0.1"></div>

main.js

/***********************
 parallax BEGIN
 ***********************/
function getCoords(elem) {
	const box = elem.getBoundingClientRect();
	return {
		top: box.top + pageYOffset,
		left: box.left + pageXOffset
	};
}

function scrollParallax(current) {
	let currentMultiplier = 0.1;
	let defaultOffset = 0;
	let windowHalf = window.innerHeight / 2;
	let elementHeight = current.offsetHeight;
	current.dataset.pos = '0';

	if (current.dataset.parallax) {
		currentMultiplier = parseFloat(current.dataset.parallax);
	}

	const make = function () {
		let normallevel = window.pageYOffset + windowHalf - elementHeight / 2;
		let currentDelta = current.dataset.pos;
		let newDelta;
		let tweenDelta;
		let posResult;
		
		defaultOffset = parseFloat(getCoords(current).top * currentMultiplier);
		newDelta = parseInt((normallevel * currentMultiplier));
		tweenDelta = (currentDelta - ((currentDelta - newDelta) * 0.05)).toFixed(1);
		posResult = tweenDelta - defaultOffset;

		current.style.transform = 'translate3d(0px, ' + posResult + 'px, 0px)';

		current.dataset.pos = tweenDelta;
		window.requestAnimationFrame(make);
	};

	if (window.requestAnimationFrame) {
		make();
	}
}

document.addEventListener('DOMContentLoaded', function () {
	if (device.desktop()) {
		const parallaxElements = document.querySelectorAll('.js-parallax');
		for (let i = 0; i < parallaxElements.length; i++) {
			scrollParallax(parallaxElements[i]);
		}
	}
});
/***********************
 parallax END
 ***********************/