Return value from nested callback instead of the parent function

Question

I have a parent function which has multiple callbacks and need to pass the result of the innermost callback to the function which calls this "parent" function. As NodeJS is asynchronous, my parent function obviously always returns before the callbacks are executed.

How can I make my callbacks return to the caller?

Code example that I am using right now -

var addNewUser = function(hash,name,number,time,syncTime){

// check here if the user exists or not by querying the phone number first
connection.query('SELECT user_id FROM users WHERE user_phone_number ="' +number+'"',function(err,rows,fields){

    if(rows[0]) return false;

    connection.query('INSERT INTO users (authorization_hash,user_name,user_phone_number,user_local_time,is_active,last_synced) VALUES ("'+hash+'","'+name+'","' + number+'","' +time+'","1","'+syncTime+'")',function(err,rows,fields){
    if(err) throw err;

    return true;    
        });

});
}

I Want to be able to return this callback return to the caller function.

Unrelated to the actual question, but the sample code is problematic in that checking whether a user exists via a SELECT query before an INSERT creates a race condition. Look into INSERT INTO ... ON DUPLICATE KEY ... or equivalent for your DB. — Cedric Han
– Cedric Han, Commented Aug 11, 2013 at 22:48
Thanks for pointing this out! Would INSERT IGNORE be equivalent ( I understand that it gives all errors as warnings). — VaibhavAggarwal
– VaibhavAggarwal, Commented Aug 11, 2013 at 23:51
A question just came up in my head, that why would I run into a race condition, since the query to insert is inside a callback, and it will be always called after the select query is completed. Please let me know If I am missing a fundamental here. — VaibhavAggarwal
– VaibhavAggarwal, Commented Aug 12, 2013 at 0:06
User A SELECT ... returns no rows. User B SELECT ... returns no rows. User A INSERT ... successfully inserts. User B INSERT ... fails. In this case you could probably inspect the error that came back from user B's INSERT statement but you would be better off with an atomic INSERT statement that does the check for you. — Cedric Han
– Cedric Han, Commented Aug 12, 2013 at 5:38

Cedric Han · Accepted Answer · 2013-08-11 22:46:36Z

1

Have addNewUser accept a callback as its last argument and have the innermost function call the callback with the value.

Alternatively, you could look into having addNewUser return a promise. RSVP or Q are implementations of the Promises/A :

function addNewUser(hash,name,number,time,syncTime) {
    var deferred = Q.defer();
    connection.query("SELECT ...", function(err, rows, fields) {
        if(err) { deferred.reject(err); }
        if(rows[0]) { deferred.reject("some reason"); }
        connection.query("INSERT INTO ...", function(err, rows, fields) {
            if(err) { deferred.reject(err); }
            deferred.resolve(rows[0]); // Whatever addNewUser would return normally
        });
    });

    return deferred.promise;
}

Then the caller would use it like this:

addNewUser(...).then(function(newUserAdded) {
    // Do something with newUserAdded here
}, function(err) {
    // Do something with the error here
});

answered Aug 11, 2013 at 22:46

Cedric Han

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1 Comment

VaibhavAggarwal Over a year ago

I ended up passing a callback as a parameter to the function, and it works exactly as I want it to! Also I realized that fundamentally I can not return a value from callback.

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