return null value in node.js?

Question

return null value in node.js?

I am new in nodejs. Here is my code in nodejs file. i want to send data from nodejs to other javascript use json.stringify,but my problem is i get null value... ----------------EDIT-----------------------

my code is

function handler ( req, res ) {
        calldb(dr,ke,function(data){
            console.log(data); //successfully return value from calldb                                      
        });
    //i think my problem bellow...
    res.write( JSON.stringify(data)); //send data to other but it's null value
    res.end('\n');
}

function calldb(Dr,Ke,callback){
    // Doing the database query
    query = connection.query("select id,user from tabel"),
        datachat = []; // this array will contain the result of our db query
    query
    .on('error', function(err) {
        console.log( err );
    })
    .on('result', function( user ) {
        datachat.push( user );
    })
    .on('end',function(){
        if(connectionsArray.length) {
            jsonStringx = JSON.stringify( datachat );
            callback(jsonStringx); //send result query to handler
        }
    });

}

How to fix this problem?

javascript
node.js

Answer 1

You will need to use callbacks, returning data directly will just return null because the end event handler is called later when all the data is ready. Try something like :

function handler ( req, res ) {
    calldb(dr, ke, function(data){
       console.log(data);
       res.write( JSON.stringify(data)); 
       res.end('\n');
    });
}

function calldb(Dr,Ke, callback) { 

    var query = connection.query('SELECT id,userfrom tabel'),
        datachat= []; // this array will contain the result of our db query

    query
     .on('error', function(err) {
        console.log( err );
     })
     .on('result', function( user ) {
        datachat.push( user );
     })
     .on('end',function() {
        callback(datachat);
    }); 

}

Answer 2

The problem is that nodejs is asynchronous. It will execute your res.write( JSON.stringify(data)); before your function will be called. You have two options: one to avoid callback:

    .on('end',function(){
      if(connectionsArray.length) {
        jsonStringx = JSON.stringify( datachat );
        res.write( JSON.stringify(data)); 
        res.end('\n');
      }
    }

the other have the response in the callback function like this:

function boxold() {
  box(function(data) {
        res.write( JSON.stringify(data)); 
        res.end('\n');
        //console.log(data);
  });
}