const backTop = () => {
    const cubic = value => Math.pow(value, 3)
    const easeInOutCubic = value => value < 0.5
    ? cubic(value * 2) / 2
    : 1 - cubic((1 - value) * 2) / 2
    const el = document.body
    const beginTime = Date.now()
    const beginValue = el.scrollTop
    const rAF = window.requestAnimationFrame || (func => setTimeout(func, 16))
    const frameFunc = () => {
        const progress = (Date.now() - beginTime) / 500
        if (progress < 1) {
            el.scrollTop = beginValue * (1 - easeInOutCubic(progress))
            rAF(frameFunc)
        } else {
            el.scrollTop = 0
        }
    };
    rAF(frameFunc)
}