mounted() {
         
           
                // let _this=this
                window.addEventListener('scroll', () => {
                    let top = document.documentElement.scrollTop || document.body.scrollTop || window.pageYOffset
                    console.log(top);
                    if (top > 700) {
                        this.scrollTag = true;
                    } else {
                        this.scrollTag = false
                    }

                })

        }