Why does my function lose its return value?

Question

Good afternoon, I'm now learning nodeJS. Right now I have a problem with a function I devised and I can't seem to find the problem

var http = require('http');
var mysql = require('mysql');
var url = require('url');
http.createServer(function (req, res) {

if (req.url === '/favicon.ico') {
    //res.writeHead(200, {'Content-Type': 'image/x-icon'} );
    res.end();
    //console.log('favicon requested');
    return;
  }

var con = mysql.createConnection({
host: "localhost",
user: "admin",
password: "admin",
database: "brandibDB"
});
res.writeHead(200, {'Content-Type': 'text/plain'});
var addr = url.parse(req.url, true);
var teste = addr.pathname.split('/');
var tester = getJson(teste,con);
console.log(tester);
//console.log(teste);
//res.write(teste[1] + teste[2]);

res.end();

}).listen(8080);

function getJson(teste,con){
var resultado = "";

con.connect(function(err){
    if (err) throw err;

    if(teste[1] == 'users'){
        if(teste.length!=2){
            var str = "SELECT * FROM tblUsers where id ="+ mysql.escape(teste[2]) + "";
            con.query(str, function (err, result, fields) {
                if (err) throw err;
                resultado = JSON.stringify(result);
              });
        }
        else{
            var str = "SELECT * FROM tblUsers";
            con.query(str, function (err, result, fields) {
                if (err) throw err;
                resultado = JSON.stringify(result);
            });
        }
    }

});

con.end;
return resultado;
}

console.log('Isto deve ser uma consola');

right now I have data in a db and I'm testing accessing this server with url: "localhost:8080/users/1" or "localhost:8080/users/" When I run the second link for example, it should go into the function getJson and return the right value(a json with all the user registries) but it just returns empty. I've tried putting console logs inside the function around the lines "resultado = Json..." and it displays the right value. The result is lost afterwards.

Any tips?

score -1 · Accepted Answer · answered Mar 01 '19 at 16:26

It's because your getJson function has returned before the asynchronous callback from the query has completed, therefore resultado is still an empty string.

Instead you could either provide a callback to getJson that gets called when the query returns, or better still, use promises. Something like this:

  var tester = getJson(teste, con)
  .then(result => {
    console.log('the results were:', results)
  })
  .catch(err => {
    console.log('Something went wrong', err);
  });

and your function becomes something like:

function getJson(teste, con) {
  return new Promise((resolve, reject) => {
    con.connect(function (err) {
      if (err) throw err;

      if (teste[1] == 'users') {
        if (teste.length != 2) {
          var str = "SELECT * FROM tblUsers where id =" + mysql.escape(teste[2]) + "";
          con.query(str, function (err, result, fields) {
            if (err) return reject(err);
            return resolve(JSON.stringify(result));
          });
        } else {
          var str = "SELECT * FROM tblUsers";
          con.query(str, function (err, result, fields) {
            if (err) return reject(err);
            return resolve(JSON.stringify(result));
          });
        }
      }
    });  
  })
}

I'd also recommend ditching var and using let and const, it's much easier to keep track of your variable scopes that way.

Why does my function lose its return value?

1 Answers1