实现元素拖拽

以Vue为例：

 initDrag() {
   const box = document.getElementsByClassName('fast-menu-container')[0] // 获取元素
   const box_inner = document.querySelector('.fast-menu-bar')
   const d_max = box_inner.offsetWidth - box.offsetWidth // 可移动最大距离

   let startX // 鼠标相对与div左边，上边的偏移
   box.onmousedown = (e = window.event) => {
     startX = e.clientX
     this.isDragging = true // 设为true表示可以移动
   }

   document.onmousemove = (e = window.event) => {
     if (!this.isDragging) return

     const moveX = e.clientX - startX // 得到距离左边移动距离
     const _mname = moveX >= 0 ? 'min' : 'max'
     const _max = moveX >= 0 ? d_max : d_max * -1
     const scroll = Math[_mname](moveX, _max) * -1
     box.scrollLeft += scroll / 8
   }

   document.onmouseup = () => {
     this.isDragging = false // 设置为false不可移动
   }
 }