动态设置顶部栏，滚动透明，颜色变换（js）

    window.onscroll = (event) => {
            let realHeight = document.documentElement.scrollTop || document.body.scrollTop;
            let optatic = 0.8 * (realHeight/142);
            if(optatic <= 0.8 ) {
                this.setState({
                    bg: `rgba(234, 44, 44, ${optatic})`,
                })
            }
        }