js中判断输入的验证码是否正确

// 生成一个4位随机验证码
function generateCaptcha() {
    const chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
    let captcha = '';
    for (let i = 0; i < 4; i++) {
        captcha += chars.charAt(Math.floor(Math.random() * chars.length));
    }
    return captcha;
}

// 存储验证码到sessionStorage（仅用于演示，实际应用中应存储在服务器端）
sessionStorage.setItem('captcha', generateCaptcha());

<input type="text" id="userCaptcha" placeholder="请输入验证码">
<button onclick="validateCaptcha()">验证</button>

function validateCaptcha() {
    const userInput = document.getElementById('userCaptcha').value;
    const storedCaptcha = sessionStorage.getItem('captcha');

    if (userInput === storedCaptcha) {
        alert('验证码正确！');
    } else {
        alert('验证码错误，请重试！');
        // 重新生成并显示新的验证码
        sessionStorage.setItem('captcha', generateCaptcha());
    }
}