上传文件前验证文件扩展名

function validateFileExtension(file) {
    const allowedExtensions = ['jpg', 'jpeg', 'png', 'gif'];
    const fileExtension = file.name.split('.').pop().toLowerCase();
    if (!allowedExtensions.includes(fileExtension)) {
        alert('只允许上传以下格式的文件：' + allowedExtensions.join(', '));
        return false;
    }
    return true;
}

document.getElementById('fileInput').addEventListener('change', function(event) {
    const file = event.target.files[0];
    if (file && validateFileExtension(file)) {
        // 继续上传操作
    }
});

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

const storage = multer.diskStorage({
    destination: function (req, file, cb) {
        cb(null, 'uploads/');
    },
    filename: function (req, file, cb) {
        cb(null, Date.now() + path.extname(file.originalname));
    }
});

const upload = multer({
    storage: storage,
    fileFilter: function (req, file, cb) {
        const allowedExtensions = ['jpg', 'jpeg', 'png', 'gif'];
        const fileExtension = path.extname(file.originalname).toLowerCase().slice(1);
        if (!allowedExtensions.includes(fileExtension)) {
            return cb(new Error('只允许上传以下格式的文件：' + allowedExtensions.join(', ')));
        }
        cb(null, true);
    }
});

app.post('/upload', upload.single('file'), function (req, res) {
    res.send('文件上传成功');
});

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