Why is this function's return value ignored?

Question

Why is this function's return value ignored?

I'm fairly sure the problem here is simply I'm not thinking in the necessary, asynchronous way yet for Node.js.

I have a function userExists() in a module db which checks whether a user is in a database when passed an email, and returns true or false.

The relevant part:

userExists : function(email) {
    var client = new pg.Client(app.conString);
    var query;
    var result = false;

    client.connect();

    query = client.query(
        "SELECT * FROM users WHERE email = $1",
        [email]
    );

    query.on('row', function(row) {
        result = true;
    });

    query.on('end', function() {
        client.end();
        return result;
    });

I am calling it like this:

if (!db.userExists(req.session.email)) {
        newUser();
    } else {
        updateUser();
    }
}

It is always newUser() which is called, whatever userExists() returns. How can I solve this?

node.js
asynchronous

Answer 1

You got the time wrong. The queries finishes after you function is run, so you need to pass a callback function and "wait" for the result.

userExists = function(email, cb) {
    var client = new pg.Client(app.conString);
    var query;
    var result = false;

    client.connect();

    query = client.query(
        "SELECT * FROM users WHERE email = $1",
        [email]
    );

    query.on('row', function(row) {
        result = true;
    });

    query.on('end', function() {
        client.end();
        cb(result);
    });
}


// Time 1: ask is the user exists
db.userExists(req.session.email, function(exists) {
    // Time 3: Yeah result came back
    if (exists) {
        updateUser()
    }
    else {
        newUser();
    }
});
// Time 2: I've asked, let's wait for the result
// The following code will likely be executed before you get the result back