Node Callback from database query to express request

Question

I want the function below (listServers()) to callback data to be used in the express request below. But I am confused by the set up of a callback as I have got conflicting information from searching.

The server runs listServers fine but seems to timeout when returning the value to the request.

The output from me requesting / on the server is:

forEach length:1
forEach length:2
listServers length:2
::ffff:*IP* - - [30/Nov/2016:09:31:19 +0000] "GET / HTTP/1.1" - -

Code:

var listServers = function (err, data) {
    var list=[];
    db.all("SELECT * FROM Servers;", function(err, rows){
    rows.forEach(function(e){
      var confi = fs.readFileSync(paths.factorioDir + "server" + e.serverID + paths.conf);
      var conf = JSON.parse(confi);
      var item = {id: e.serverID, conf:conf};
      list.push(item);
        console.log("forEach length:" + list.length);
    });
    if (err) throw err;
      console.log("listServers length:" + list.length);
      data = list;
    return data;
    });

};

admin.get('/', function(req, res) {
   listServers(function(err, data){
     console.log(data.length);
     servers = data;
     console.log("/ servers length"+ servers.length);
      var adminTemplate = pug.compileFile(__dirname + '/template.pug');
      var context = { servers: servers };
      var html = adminTemplate(context);
      res.send(html);
});
});

Full code is here

http://javascriptissexy.com/understand-javascript-callback-functions-and-use-them/ — xShirase, Nov 30 '16 at 09:45

score 0 · Accepted Answer · answered Nov 30 '16 at 09:55

What you are passing to listServers() is a callback function, so instead of returning the data/throwing an error in listServers() you should invoke that callback and pass it either the error or the data:

var listServers = function (callback) {
    var list=[];
    db.all("SELECT * FROM Servers;", function(err, rows){
        if (err) {
            callback(err, []);

            return;
        }

        rows.forEach(function(e){
            var confi = fs.readFileSync(paths.factorioDir + "server" + e.serverID + paths.conf);
            var conf = JSON.parse(confi);
            var item = {id: e.serverID, conf:conf};
            list.push(item);
            console.log("forEach length:" + list.length);
        });

        console.log("listServers length:" + list.length);
        callback(null, list);
    });
};

admin.get('/', function(req, res) {
    listServers(function(err, data){
        if (err) {
            // Handle error
        }

        console.log(data.length);
        servers = data;
        console.log("/ servers length"+ servers.length);
        var adminTemplate = pug.compileFile(__dirname + '/template.pug');
        var context = { servers: servers };
        var html = adminTemplate(context);
        res.send(html);
    });
});

Node Callback from database query to express request

1 Answers1