输入无效的城市会导致UI中什么都不发生，并在控制台中出现错误

function validateCity(city) {
    const validCities = ["北京", "上海", "广州", "深圳"]; // 示例有效城市列表
    if (!validCities.includes(city)) {
        alert("无效的城市名称");
        return false;
    }
    return true;
}

document.getElementById("cityInput").addEventListener("blur", function() {
    const city = this.value;
    if (!validateCity(city)) {
        this.value = ""; // 清空输入框
    }
});

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

app.use(express.json());

app.post('/validateCity', (req, res) => {
    const city = req.body.city;
    const validCities = ["北京", "上海", "广州", "深圳"]; // 示例有效城市列表
    if (!validCities.includes(city)) {
        return res.status(400).json({ error: "无效的城市名称" });
    }
    res.json({ message: "城市名称有效" });
});

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