回到顶部 超过一定高度后显示

（1）mouted

mounted() {

            // 事件监听滚动条

            window.addEventListener('scroll', this.watchScroll)

        },

（2）methods

methods: {

            // 回到顶部

            goTop() {

                document.body.scrollTop = document.documentElement.scrollTop = 0;

                this.showTop = false

            },

            // 回到顶部 屏幕滑动值判断

            watchScroll() {

                var scrollTop = window.pageYOffset || document.documentElement.scrollTop || document.body

                    .scrollTop; // 当滚动超过 100 时，回到顶部显示 

                if (scrollTop > 100) {

                    this.go_top_flag = false

                } else {

                    this.go_top_flag = true

                }

            },

        }