Return function Javascript

Question

I have several problems with return of queries.

Here, what I would like to do :

//If the email hasn't a good format
if(email_not_good_format())
   //I do something
else if(email_already_exists_in_mysql(email))
  //I do something


function email_already_exists_in_mysql(email){
connection.query('SELECT COUNT(*) AS nb FROM user WHERE emailUser = ' + connection.escape(email), function(err, rows, fields) {
        if (err) throw err;
        if(rows[0].nb == 0)
            return false;
        else
            return true;
    });
}

I saw on different posts callback function but it doesn't work for what I want to do.

for async work on JS take a look to promises. You have the Q library for node.js. — TMichel, Jul 23 '13 at 11:20
The call to `.query` is *asynchronous*. That's why you have to provide a callback. Have a look at this question to understand the problem and for a potential solution: http://stackoverflow.com/q/14220321/218196 (even though it's about Ajax, the same principals apply). — Felix Kling, Jul 23 '13 at 11:30
You're attempting to perform a synchronous set of actions using an asynchronous language. You might like to start with some tutorials on JavaScript before attempting something a little more complex like database connections. Have you looked at any JavaScript tutorials? — PP., Jul 23 '13 at 12:15

score 1 · Accepted Answer · answered Jul 23 '13 at 12:52

I saw on different posts callback function but it doesn't work for what I want to do.

Yes it does, you just need to change the way you think about code. Instead of writing email_already_exists_in_mysql you should instead write a function called if_email_already_exists_in_mysql:

/* Executes callback if email
 * already exists in mysql:
 */
function if_email_already_exists_in_mysql (email,callback) {
    connection.query(
        'SELECT COUNT(*) AS nb FROM user WHERE emailUser = ' +
        connection.escape(email),

        function(err, rows, fields) {
            if(rows[0].nb != 0) {
                callback();
            }
        }
    )
}

Then instead of writing this:

//If the email hasn't a good format
if(email_not_good_format()) {
    //I do something
}
else if(email_already_exists_in_mysql(email)) {
    //I do something
}

you write it like this instead:

//If the email hasn't a good format
if(email_not_good_format()) {
    //I do something
}
else {if_email_already_exists_in_mysql(email),function(){
    //I do something
})}

Now, you may ask yourself, what if there is another else after that? Well, you need to modify the if_email_already_exists_in_mysql function to behave like and if...else instead of just and if:

function if_email_already_exists_in_mysql (email,callback,else_callback) {
    connection.query(
        'SELECT COUNT(*) AS nb FROM user WHERE emailUser = ' +
        connection.escape(email),

        function(err, rows, fields) {
            if(rows[0].nb != 0) {
                callback();
            }
            else if(else_callback) {
                else_callback();
            }
        }
    )
}

so that you can call it like this:

//If the email hasn't a good format
if(email_not_good_format()) {
    //I do something
}
else {
    if_email_already_exists_in_mysql(email),function(){
        //I do something
    },
    // else
    function(){
        //I do something else
    }
)}

You can write async code to do pretty much anything regular code can do only instead of returning a value you pass in a callback. Remember:

return in synchronous code == passing in callbacks in asynchronous code.

The code structure must therefore be different but as I demonstrated above the logic you want to implement can be exactly the same.

Return function Javascript

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