js to realize a simple advertising window

<style>
        .box {
            width: 180px;
            height: 180px;
            background: #f0f0f0;
            position: absolute;
        }
 
        .X {
            width: 30px;
            height: 30px;
            background: #eaeaea;
            color: firebrick;
            text-align: center;
            line-height: 30px;
        }
</style>

<div class="box">
        <div class="X">X</div>
</div>

<script>
        //Get the node let boxObj = document.querySelector('.box');
        let xObj = document.querySelector('.X');
 
        //Get the position of box let boxLeft = boxObj.offsetLeft;
        let boxTop = boxObj.offsetTop;
        // Bind X
 
        xObj.onclick = clickFn;
        xObj.onmouseover = overFn;
 
        //Move the mouse in and change to hand shapefunction overFn() {
            xObj.style.cursor = 'pointer';
        }
        let num=0;
        //When the mouse clicks X, the window will not be cancelled but will jump to another random position. function clickFn() {
            boxObj.style.left = boxLeft + rand(1, 1000) + 'px';
            boxObj.style.top = boxTop + rand(1, 500) + 'px';
            num++;
            if(num==20){
                boxObj.style.display='none';
            }
        }
        //Random number function rand(min, max) {
            return Math.round(Math.random() * (max - min) + min);
        }
</script>