只允许在指定范围里拖拽

let oDiv = document.querySelector('div')
    let flag = false;
    oDiv.addEventListener('mousedown',(e)=>{
        let x = e.offsetX;
        let y = e.offsetY;
        flag = true;
        document.addEventListener('mousemove',(event)=>{
            let _w = window.innerWidth - oDiv.offsetWidth;
            let _h = window.innerHeight - oDiv.offsetHeight;
            let div_left = event.clientX - x;
            let div_top = event.clientY - y;
            div_top = Math.min(Math.max(0,div_top),_h);
            div_left = Math.min(Math.max(0,div_left),_w);
            if(flag){
              oDiv.style.left = div_left + 'px';
              oDiv.style.top = div_top + 'px';
            }
            
        })
    })
    oDiv.addEventListener('mouseup',()=>{
      flag = false;
    })