Quick'n'dirty parallax for a bunch of stacked layers

parallax.js

MAX_TRANSLATE = 6;

const elements = [
    "background", "clouds", "back-mountain", "front-mountain",
    "hills-4", "hills-3", "hills-2", "hills-1", "birds"
].map(klass => document.querySelector(`.${klass}`));


function updatePositions(height, width) {
    const maxVHTranslation = -height * MAX_TRANSLATE;
    const maxVWTranslation = -width * MAX_TRANSLATE;
    const length = elements.length;
    for (let i=0; i<elements.length; i++) {
        const translateVH = i / length * maxVHTranslation;
        const translateVW = i / length * maxVWTranslation;

        elements[i].style.transform = 
            `translate3d(${translateVW}vh, ${translateVH}vh, 0)`;
    }
}