Question

I get a client is not defined error out of the following code:

var mysql = require('mysql');
var conf = {
  'database':'database',
  'user':'user',
  'password':'password'
};

function mysqlQuery(mysql, conf, query, callback) {
  var client = mysql.createClient({
    user:conf.user,
    password:conf.password
  });
  client.query('USE ' + conf.database, function () {
    client.query(query, callback);
  });
}

mysqlQuery(
  mysql,
  conf,
  'SELECT * FROM users;',
  function selectCb(err, results) {
    if (err) {
      throw err;
    }
    console.log(results);
    client.end();
  }
);

How can I pass the client variable to my callback function? I have no control on how the callback function will be called as it is called by the mysql module.

Answer 1

You could bind ^[docs] client to callback. Inside the callback it will be available as this then:

client.query(query, callback.bind(client))

and

this.end();

You could also pass it as first parameter:

client.query(query, callback.bind(null, client))

or

client.query(query, function(err, results) {
    callback(client, err, results);
});

where callback is defined as

function selectCb(client, err, results) {..};