Mousewheel event in modern browsers

Clean and simple:

window.addEventListener("wheel", event => console.info(event.deltaY));

Browsers may return different values for the delta (for instance, Chrome returns +120 (scroll up) or -120 (scroll down). A nice trick to normalize it is to extract its sign, effectively converting it to +1/-1:

window.addEventListener("wheel", event => {
    const delta = Math.sign(event.deltaY);
    console.info(delta);
});

Reference: MDN.