回调 function 内部的变量集无法在外部访问

Question

I am setting a variable inside the MySQL query result callback function so that I can access it later, but if I try to do if(!data) in the next step it says data is not defined.

await connection.query('SELECT * FROM `licenses` WHERE `licensekey` = ?', [key], function (error, results, fields) {
      let data = results[0]
    })

The key variable is defined properly and I am able to do console.log(results[0]) inside of that function.

Answer 1

There is a callback passed to .query function. await won't be waiting for callback to be executed here.

Try wrapping it inside Promise as below

const data = await new Promise((resolve, reject) => {
  connection.query('SELECT * FROM `licenses` WHERE `licensekey` = ?', [key], function (error, results, fields) {
    if (error) reject(error);
    else resolve(results[0]);
  });
});

if(!data) {
  // ...
}