检查Node.js中的MySql数据库中是否已存在用户

const mysql = require('mysql');

const connection = mysql.createConnection({
  host: 'localhost', // 数据库主机名
  user: 'root', // 数据库用户名
  password: 'password', // 数据库密码
  database: 'mydatabase' // 数据库名称
});

connection.connect((err) => {
  if (err) throw err;
  console.log('Connected to MySql database!');
});

function checkUserExists(username) {
  return new Promise((resolve, reject) => {
    const query = `SELECT * FROM users WHERE username = '${username}'`;
    connection.query(query, (err, results) => {
      if (err) reject(err);
      resolve(results.length > 0);
    });
  });
}

const username = 'john.doe';

checkUserExists(username)
  .then((exists) => {
    if (exists) {
      console.log('User already exists!');
    } else {
      console.log('User does not exist!');
    }
  })
  .catch((err) => {
    console.error(err);
  });