scrollWindowBy

function scrollWindowBy(value: number, duration: number, easingFunc: (t: number) => number) {
    let startTime: number
    const startPos: number = window.scrollY
    const clientHeight: number = window.innerHeight
    const maxScroll = document.body.scrollHeight - clientHeight;
    const scrollIntendedDestination = startPos + value;
    // low and high bounds for possible scroll destinations
    const scrollEndValue = Math.min(Math.max(scrollIntendedDestination, 0), maxScroll)
    // create recursive function to call every frame
    const scroll = (timestamp: number) => {
        startTime = startTime || timestamp
        const elapsed = timestamp - startTime
        window.scrollTo(0, startPos + (scrollEndValue - startPos) * easingFunc(elapsed / duration))
        elapsed <= duration && window.requestAnimationFrame(scroll)
    }
    // call recursive function
    if (startPos != scrollEndValue) window.requestAnimationFrame(scroll)
}

const easeInOutCubic = (t: number) => {
    return t < .5 ? 4 * t * t * t : (t - 1) * (2 * t - 2) * (2 * t - 2) + 1
}

scrollWindowBy(120, 1000, easeInOutCubic)