js 用户名已存在

document.getElementById('username').addEventListener('input', function() {
    const username = this.value;
    fetch(`/checkUsername?username=${username}`)
        .then(response => response.json())
        .then(data => {
            if (data.exists) {
                document.getElementById('username-error').innerText = '用户名已存在';
            } else {
                document.getElementById('username-error').innerText = '';
            }
        });
});

app.get('/checkUsername', async (req, res) => {
    const username = req.query.username;
    const exists = await checkUsernameInDatabase(username);
    res.json({ exists });
});

async function checkUsernameInDatabase(username) {
    // 假设使用MongoDB
    const user = await User.findOne({ username });
    return user !== null;
}

CREATE UNIQUE INDEX idx_username ON users(username);