Node.js + socket.io + MySQL correction of syntax - node.js

Question

Considering that my server.js looks almost like this. Just send you the relevant part. I did not receive anything from the query, I do have data in the database, and "sendNotification" is triggered by the jQuery function in the client. Everything works and since var notis = []; returns an empty value and is what is shows as response. I know I have to debug SQL and that's what I'm going to do but anyway want to be sure of this other things. So my questions are:
1) Is a right syntax for node.js, considering this async behavior? (which I still don't understand )
2) The query always should be inside of the "io.sockets.on('connection')" part?
connection = mysql.createConnection({
host: 'localhost',
user: '',
password: "",
database: 'table' //put your database name
}),
...
connection.connect(function(err) {
// connected! (unless `err` is set)
console.log(err);
});
…
var sqlquery = function(uID,vs){
var notis = [];
connection.query("SELECT * FROM notification WHERE kid = ? AND v = ? ORDER BY id DESC",[uID,vs])
.on("result", function (data){
return notis.push(data);
});
};
io.sockets.on('connection', function(socket) {
...
socket.on("sendNotification", function(data) {
var roomBName = data.room_name.replace("room-",""),
found = [];
var roomSelected = _.find(rooms, function (room) { return room.id == roomBName });
for (var person in people) {
for (var i = 0, numAttending = roomSelected.peopleAttending.length; i < numAttending; i++) {
if (people[person].name == roomSelected.peopleAttending[i]) {
found.push(person);
}
}
}
for (var i = 0, numFound = found.length; i < numFound; i++) {
**result = sqlquery(9,2);**
io.to(found[i]).emit('notification', result);
};
});

Answer 1

Your sqlquery() function will not accomplish anything useful. Because connection.query() is asynchronous, that means it provides the response sometime LATER after sqlquery() has already finished.
The only way in node.js to use an async result is to actually use it in the callback that provides it. You don't just stuff it into some other variable and expect the result to be there for you in other code. Instead, you use it inside that callback or you call some other function from the callback and pass it the data.
Here's one way, you could change your sqlquery() function:
var sqlquery = function(uID, vs, callback){
connection.query("SELECT * FROM notification WHERE kid = ? AND v = ? ORDER BY id DESC",[uID,vs])
.on("result", function (data){
callback(null, data);
});
// need to add error handling here if the query returns an error
// by calling callback(err)
};
Then, you could use the sqlquery function like this:
found.forEach(function(person, index) {
sqlquery(..., function(err, result) {
if (err) {
// handle an error here
} else {
io.to(person).emit('notification', result);
}
});
});
And, it looks like you probably have similar async issues in other places too like in connection.connect().

Answer 2

In addition to #jfriend00, this could be done with new ES6 feature Promise :
var sqlquery = function(uID, vs){
return new Promise(function(resolve, reject){
connection.query("SELECT * FROM notification WHERE kid = ? AND v = ? ORDER BY id DESC",[uID,vs])
.on("result", function (data){
resolve(data);
});
});
};
Now you can use it like :
found.forEach(function(person, index) {
sqlquery(...)
.then(function(result){
io.to(person).emit('notification', result);
});
});