maybe missing a trick with javascript coding - variable handling (socket.io example)

Question

I would finally like to reach some clarity on this aspect of programming, being basically self-taught. It's to do with passing variables around between functions.

With the code below, data.roomId is not recognized in the first function (obviously). I could place the function inside the joinRoom function to make it recognize data.roomId, but then how does the leaveRoom function recognize sendHeartbeat?

`io.on('connection', function (socket) {
    function sendHeartbeat(){
        setTimeout(sendHeartbeat, 8000);
        socket.broadcast.to(data.roomId).emit('ping', { beat : 1 });
    }
    socket.on('joinRoom', function (data) {
        socket.join(data.roomId)
        setTimeout(sendHeartbeat, 8000);
    });
    socket.on('leaveRoom', function (data) {
        socket.leave(data.roomId)
        clearTimeout(sendHeartbeat)
    });
});`

sendHeartbeat is defined within the scope of function(callback) - so it is in scope, therefore (as you put it) "recognized" — Jaromanda X
– Jaromanda X, Commented Sep 21, 2017 at 23:39

Francois · Accepted Answer · 2017-09-21 23:45:11Z

1

You can do what we call a closure, it is a function that returns a function keeping track of local variables. It is a cool concept !

function sendHeartbeat(roomId){
    return function() {
        socket.broadcast.to(roomId).emit('ping', { beat : 1 });
    }
}
socket.on('joinRoom', function (data) {
    socket.join(data.roomId)
    setTimeout(sendHeartbeat(data.roomId), 8000);
});

You call the function sendHeartbeat passing it the roomId, and it returns the function you want to be executed in your setTimeout :)

edited Sep 21, 2017 at 23:45

answered Sep 21, 2017 at 23:39

Francois

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1 Comment

MikeyB Over a year ago

Ok, good to know! Guess I'll test it in future but right now I've moved away from pinging to keep a socket.io connection open and went with setting session cookies using npmjs.com/package/express-socket.io-session

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