detects vertical scroll movement on a page/element, works on mobile and desktop

scrollDetector.js

function getTouch(event) {
    if (event.touches) return event.touches[0]
    return event;
}

function addSwipeListener(element, callback, startCallback, finishCallback) {
    let y;

    let handleTouchStart = (event) => {
        y = getTouch(event).clientY;

        if (startCallback) startCallback();
    }

    let handleTouchMove = (event) => {
        if (!y) return;

        let newY = getTouch(event).clientY;
        let delta = y - newY;
        y = newY;

        if (delta != 0) callback(delta);
    }

    let handleTouchEnd = () => {
        y = undefined;
        if (finishCallback) finishCallback();
    }

    element.addEventListener('touchstart', handleTouchStart, false);
    element.addEventListener('mousedown', handleTouchStart, false);
    element.addEventListener('touchmove', handleTouchMove, false);
    element.addEventListener('mousemove', handleTouchMove, false);
    element.addEventListener('touchend', handleTouchEnd, false);
    document.addEventListener('mouseup', handleTouchEnd, false);
}

export default function onScroll(element, callback){
    let currentDelta = 0;
    let ignoreScroll = false;

    let call = () => {
        if(currentDelta > 0)
            callback(100);
        else
            callback(-100);

        currentDelta = 0;
    }

    let tryCall = (val = 99) => {
        if(currentDelta > val || currentDelta < -val){
            call();
            return true;
        } else {
            return false;
        }
    }

    addSwipeListener(element, (delta) => {
        if(ignoreScroll){
            return;
        }

        currentDelta += delta;

        if(tryCall()){
            ignoreScroll = true;
        }

    }, null, () => {
        if(!ignoreScroll){
            tryCall(40);
        }

        ignoreScroll = false;
        
    });
    element.addEventListener('wheel', e => callback(e.deltaY));
}