如何使用JavaScript即时发送电子邮件？

<form id="emailForm">
  <label for="to">收件人邮箱：</label>
  <input type="email" id="to" required><br>
  <label for="subject">主题：</label>
  <input type="text" id="subject" required><br>
  <label for="message">正文：</label>
  <textarea id="message" required></textarea><br>
  <button type="submit">发送邮件</button>
</form>

<script>
  document.getElementById('emailForm').addEventListener('submit', function(e) {
    e.preventDefault();
    
    var to = document.getElementById('to').value;
    var subject = document.getElementById('subject').value;
    var message = document.getElementById('message').value;
    
    var data = {
      to: to,
      subject: subject,
      message: message
    };
    
    fetch('/sendEmail', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json'
      },
      body: JSON.stringify(data)
    })
    .then(function(response) {
      if (response.ok) {
        alert('邮件发送成功！');
      } else {
        alert('邮件发送失败！');
      }
    })
    .catch(function(error) {
      console.error('邮件发送失败：', error);
    });
  });
</script>

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

const app = express();
app.use(express.json());

app.post('/sendEmail', function(req, res) {
  var to = req.body.to;
  var subject = req.body.subject;
  var message = req.body.message;
  
  var transporter = nodemailer.createTransport({
    host: 'your-smtp-server',
    port: 587,
    secure: false,
    auth: {
      user: 'your-username',
      pass: 'your-password'
    }
  });
  
  var mailOptions = {
    from: 'your-email@example.com',
    to: to,
    subject: subject,
    text: message
  };
  
  transporter.sendMail(mailOptions, function(error, info) {
    if (error) {
      console.error('邮件发送失败：', error);
      res.status(500).send('邮件发送失败');
    } else {
      console.log('邮件发送成功：', info.response);
      res.sendStatus(200);
    }
  });
});

app.listen(3000, function() {
  console.log('服务器已启动，监听端口3000');
});