index.js

let calculateAngle = function (e, item, parent) {
    let dropShadowColor = `rgba(0, 0, 0, 0.3)`
    if (parent.getAttribute('data-filter-color') !== null) {
        dropShadowColor = parent.getAttribute('data-filter-color');
    }

    parent.classList.add('animated');
    // Get the x position of the users mouse, relative to the button itself
    let x = Math.abs(item.getBoundingClientRect().x - e.clientX);
    // Get the y position relative to the button
    let y = Math.abs(item.getBoundingClientRect().y - e.clientY);

    // Calculate half the width and height
    let halfWidth = item.getBoundingClientRect().width / 2;
    let halfHeight = item.getBoundingClientRect().height / 2;

    // Use this to create an angle. I have divided by 6 and 4 respectively so the effect looks good.
    // Changing these numbers will change the depth of the effect.
    let calcAngleX = (x - halfWidth) / 12;
    let calcAngleY = (y - halfHeight) / 14;

    let gX = (1 - (x / (halfWidth * 2))) * 100;
    let gY = (1 - (y / (halfHeight * 2))) * 100;

    item.querySelector('.glare').style.background = `radial-gradient(circle at ${gX}% ${gY}%, rgb(199 198 243), transparent)`;
    // And set its container's perspective.
    parent.style.perspective = `${halfWidth * 6}px`
    item.style.perspective = `${halfWidth * 6}px`

    // Set the items transform CSS property
    item.style.transform = `rotateY(${calcAngleX}deg) rotateX(${-calcAngleY}deg) scale(1.04)`;
    parent.querySelector('.inner-card-backface').style.transform = `rotateY(${calcAngleX}deg) rotateX(${-calcAngleY}deg) scale(1.04) translateZ(-4px)`;

    if (parent.getAttribute('data-custom-perspective') !== null) {
        parent.style.perspective = `${parent.getAttribute('data-custom-perspective')}`
    }

    // Reapply this to the shadow, with different dividers
    let calcShadowX = (x - halfWidth) / 3;
    let calcShadowY = (y - halfHeight) / 6;

    // Add a filter shadow - this is more performant to animate than a regular box shadow.
    item.style.filter = `drop-shadow(${-calcShadowX}px ${-calcShadowY}px 15px ${dropShadowColor})`;
}

document.querySelectorAll('.card').forEach(function (item) {
    if (item.querySelector('.flip') !== null) {
        item.querySelector('.flip').addEventListener('click', function () {
            item.classList.add('flipped');
        });
    }
    if (item.querySelector('.unflip') !== null) {
        item.querySelector('.unflip').addEventListener('click', function () {
            item.classList.remove('flipped');
        });
    }
    item.addEventListener('mouseenter', function (e) {
        calculateAngle(e, this.querySelector('.inner-card'), this);
    });

    item.addEventListener('mousemove', function (e) {
        calculateAngle(e, this.querySelector('.inner-card'), this);
    });

    item.addEventListener('mouseleave', function (e) {
        let dropShadowColor = `rgba(0, 0, 0, 0.3)`
        if (item.getAttribute('data-filter-color') !== null) {
            dropShadowColor = item.getAttribute('data-filter-color')
        }
        item.classList.remove('animated');
        item.querySelector('.inner-card').style.transform = `rotateY(0deg) rotateX(0deg) scale(1)`;
        item.querySelector('.inner-card-backface').style.transform = `rotateY(0deg) rotateX(0deg) scale(1.01) translateZ(-4px)`;
        item.querySelector('.inner-card').style.filter = `drop-shadow(0 10px 15px ${dropShadowColor})`;
    });
})