javascript - Nodejs asynchronous database function needs synchronous answer

Question

I am new to nodejs and am writing some code that needs to query my MySQL database and return a username from a given user_id. I've been reading that all your functions should be asynchronous. In this case, ideally I would like the server to be able to respond to other event requests while this query is taking place. However, it isn't a particularly large query and only returns a single value. Maybe I should make it synchronous? (If that is your answer, sample code to change it would be great) Anyways, here is my function. It gives an error near the last line "return current_username;" because current_username is undefined at that point. Any suggestions?

function get_current_username(current_user_id) {
    console.log(' | Entered get_current_username');
    sqlq = 'SELECT username FROM users WHERE id = ' + current_user_id;
    connection.query(sqlq, function(err, rows, fields) {
      if (err) throw err;
      var current_username = rows[0].username;
      console.log(' | the current_username =' + current_username);        
    });

return current_username;

}

Answer 1

Pass in a callback function to get_current_username and then call that callback function from inside of connect.query's callback:

function get_current_username(current_user_id, callback) {
    console.log(' | Entered get_current_username');
    sqlq = 'SELECT username FROM users WHERE id = ' + current_user_id;
    connection.query(sqlq, function(err, rows, fields) {
      if (err) throw err;
      var current_username = rows[0].username;
      console.log(' | the current_username =' + current_username);        
      callback(current_username);
    });
}

When you go to use this function then, you'd do something like:

get_current_username(12345, function(username) {
   console.log("I am " + username);
});

You could also check out the use of promises/futures. I have a feeling I won't be able to do an explanation of these justice, so I'll link off to this StackOverflow question about understanding Promises.

This is an architectural decision though - some people would prefer to use callbacks, especially if writing a module intended for re-use by 3rd parties. (And in fact, it's probably best to get your head fully wrapped around callbacks in the learning stages here, before adopting something like Promise.)