Mysql-nodejs nested queries

Question

It says that cannot read property authorid undefined in socket.broadcast line.

setInterval(function(){
    db.query("select * from `notifications`", function(err, rows){
        if (rows.length>temp){

            for (var i=temp; i<rows.length; i++){
                console.log('sending room post', rows[i].authorid);
                db.query("select `name` from `users` where `id`=?", [rows[i].authorid ], function(err, name){
                    name=name[0].name;
                    socket.broadcast.to(rows[i].authorid).emit('new notification', {
                        authorname:name,
                        dpid:rows[i].followid,
                        read:rows[i].read,
                        type:rows[i].type,
                        followstatus:1
                    }); 
                });


            }
            temp=rows.length;
        }
    })
}, 3000);

What does the console.log outputs?

satchcoder
– satchcoder

2015-02-07 17:52:14 +00:00
Commented Feb 7, 2015 at 17:52 — satchcoder
– satchcoder, Commented Feb 7, 2015 at 17:52

Catalin MUNTEANU · Accepted Answer · 2015-02-07 18:00:05Z

1

The problem is that the value of i is rows.length when using the socket.

You can fix this by creating a function to which you send the current row or send the index and use rows[index]:

setInterval(function() {
    db.query("select * from `notifications`", function(err, rows) {

        // You access the values of the current row
        // row.authorid
        // ...
        var broadcast = function(row) {
            console.log('sending room post', row.authorid);
            db.query("select `name` from `users` where `id`=?", [row.authorid], function(err, name) {
                name=name[0].name;
                socket.broadcast.to(row.authorid).emit('new notification', {
                    authorname:   name,
                    dpid:         row.followid,
                    read:         row.read,
                    type:         row.type,
                    followstatus: 1
                }); 
            });
        };

        if (rows.length>temp) {
            for (var i=temp; i<rows.length; i++) {
                broadcast(rows[i]);
            }
            temp=rows.length;
        }
    })
}, 3000);

edited Feb 7, 2015 at 18:00

answered Feb 7, 2015 at 17:53

Catalin MUNTEANU

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4 Comments

Arindam Barman Over a year ago

Thank you. It worked. But I can't seem to understand why i is being used by socket?

Catalin MUNTEANU Over a year ago

i was used by socket when you were calling socket.broadcast.to(rows[i].authorid). The value of i was higher than the length of rows so there was no element in rows[i]

Arindam Barman Over a year ago

Oh is it because the query is asynchronous?

Catalin MUNTEANU Over a year ago

For example having a list with 3 elements. rows[author_1, author_2, author_3]. You use this list in a for loop. You might think that when you first call socket.broadcast.to.. the value of i is 0 and you are using rows[0] => meaning author_1. Well this is wrong. You are actually accessing rows[3] which doesn't exist. And you already know the reason => asynchronous calls

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