js代码不运行问题？

<!DOCTYPE html>
<html lang="en">

<head>
      <meta charset="UTF-8">
      <meta name="viewport" content="width=device-width, initial-scale=1.0">
      <title>Document</title>
      <style>
            .box1 {
                  width: 300px;
                  height: 300px;
                  border: 1px solid black;
                  position: absolute;
                  left: 30px;
                  top: 30px;
            }

            .box1 img {
                  width: 100%;
                  height: 100%;
            }

            .box2 {
                  width: 600px;
                  height: 600px;
                  border: 1px solid black;
                  position: absolute;
                  left: 500px;
                  top: 30px;
            }

            .box2 img {
                  width: 100%;
                  height: 100%;
            }

            .mengceng {
                  display: none;
                  width: 70px;
                  height: 70px;
                  background-color: black;
                  position: absolute;
                  top: 0;
                  left: 0;
                  opacity: 0.5;
            }
      </style>
      <script>
            window.onload = function () {
                  var BOX1 = document.getElementById("box1");
                  var BOX2 = document.getElementById("box2");
                  var QMengceng = document.getElementById("mengceng");

                  QMengceng.onmouseover = function () {
                        QMengceng.style.display = "block";
                  }
                  QMengceng.onmouseout = function () {
                        QMengceng.style.display = "none";
                  }
            }
      </script>
</head>

<body>
      <div class="box1">
            <img src="../图片的使用/放大镜图片.jfif" alt="">
            <div class="mengceng"></div>
      </div>
      <div class="box2">
            <img src="../图片的使用/放大镜图片.jfif" alt="">
      </div>
</body>

</html>

我只是想给左边的图片设置一个鼠标移动上去就显示蒙层，移出就不显示，但是F12调试台为什么会显示

Uncaught TypeError: Cannot set property 'onmouseover' of null
at window.onload

而且功能也无法实现

首先document.getElementById()取的是id名，而不是class名

而且我觉得你思路不对，你在img下面加个div来显示蒙层？那么就算你代码运行成功，蒙层是显示在img下面，你应该用考虑用伪类选择器或者伪元素选择器。

我是直接改变div的class

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    #box1 {
      width: 300px;
      height: 300px;
      border: 1px solid black;
      position: absolute;
      left: 30px;
      top: 30px;
    }
    #box1 img {
      width: 100%;
      height: 100%;
    }
    #box2 {
      width: 600px;
      height: 600px;
      border: 1px solid black;
      position: absolute;
      left: 500px;
      top: 30px;
    }
    #box2 img {
      width: 100%;
      height: 100%;
    }
    .mengceng {
      width: 70px;
      height: 70px;
      background-color: black;
      position: absolute;
      top: 0;
      left: 0;
      opacity: 0.5;
    }
  </style>
  <script>
    window.onload = function () {
      var BOX1 = document.getElementById("box1");
      var BOX2 = document.getElementById("box2");
      var QMengceng = document.getElementById("mengceng");
      BOX1.onmouseover = function () {
        BOX1.className =  'mengceng';
      }
      BOX1.onmouseout = function () {
        BOX1.className = "";
      }
    };
  </script>
</head>
<body>
<div id="box1">
  <img src="images/ss.jpg" alt="">
</div>
<div id="box2">
  <img src="images/ss.jpg" alt="">
</div>
</body>
</html>

你没有mengceng这个对象，并且box1，box2是样式，并不是id，要给控件加上id

而且就算有mengceng这个对象，默认display为none也不会触发绑定的事件，先整清楚dom结构先吧