无法将图像上传变量设置为空

document.getElementById('uploadForm').addEventListener('submit', function(event) {
    var imageInput = document.getElementById('imageInput');
    if (!imageInput.value) {
        alert('请选择一个图像文件');
        event.preventDefault();
    }
});

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

const storage = multer.memoryStorage();
const upload = multer({ storage: storage });

app.post('/upload', upload.single('image'), (req, res) => {
    if (!req.file) {
        return res.status(400).send('没有选择图像文件');
    }
    // 处理上传的图像文件
    res.send('图像上传成功');
});

app.listen(3000, () => {
    console.log('服务器运行在 http://localhost:3000');
});

CREATE TABLE images (
    id INT AUTO_INCREMENT PRIMARY KEY,
    image_url VARCHAR(255) NULL
);

<form id="uploadForm" enctype="multipart/form-data">
    <input type="file" id="imageInput" name="image">
    <button type="submit">上传</button>
</form>