如何验证/限制附件文件大小

<input type="file" id="fileInput" accept=".jpg, .png">
<script>
  var fileInput = document.getElementById('fileInput');
  fileInput.addEventListener('change', function() {
    var file = fileInput.files[0];
    var maxSize = 5 * 1024 * 1024; // 5MB
    if (file.size > maxSize) {
      alert('文件大小超过限制');
      fileInput.value = ''; // 清空文件输入框
    }
  });
</script>

const express = require('express');
const multer = require('multer');

const app = express();
const upload = multer({
  limits: { fileSize: 5 * 1024 * 1024 } // 5MB
});

app.post('/upload', upload.single('file'), (req, res) => {
  if (req.file.size > req.file.limits) {
    return res.status(400).json({ error: '文件大小超过限制' });
  }
  // 文件大小符合要求，继续处理文件
  // ...
});

app.listen(3000, () => {
  console.log('服务器已启动');
});