Force a javascript code to be synchronous

Question

I have an asynchronous funcion (closing a socket) that I have to execute when the user leaves the page. I've tried to do it through jQuery :

$(window).unload(function() {
   socket.disconnect();
});

However, this doesn't work because the function is asynchronous and the client leaves the page as soon as it reach the end of the method, that's to say instantly and the request to disconnect the socket is not performed.

We can provide a callback executed at the end of the socket.disconnect() function execution. Maybe this could help...

Is there a way in javascript to solve this ?

Tricky. Couldn't you detect the departure on the server side and close it from there? — Mitya, Jul 31 '12 at 09:02
Already tried it, I use Node.JS and it is detected 2 minutes after the client leaves. That's why I want to try "the client way", to be more effective. What's more, Node sometimes detects false disconnections so this may be the only right way to really detect one. — Cydonia7, Jul 31 '12 at 09:04
Is this `socket.io`? FYI, usually this problem is solved by using some kind of heartbeat to detect connection close from the server side. — igorw, Jul 31 '12 at 09:08
@igorw Right but this detects it really late compared to the time when the clients really disconnect. — Cydonia7, Jul 31 '12 at 09:09
Not sure how Socket.IO works but that seems pointless, the socket should disconnect on page change, the connection is no longer open. I wrote my own WebSocket server so I had the ability to do this. — Sphvn, Jul 31 '12 at 09:18
That seems to be a bug in fact. How did you handle this with your server ? — Cydonia7, Jul 31 '12 at 09:20

score 3 · Accepted Answer · answered Jul 31 '12 at 09:11

3

I'm going to stick my neck out and say that I don't believe this is viable - at least not cross-browser.

As one suggested, you can interrupt navigation via onbeforeunload but this is not implemented the same cross-browser, and even if you can interrupt it, you can't (I believe) capture where the user was going, such that, in your socket closure callback, you could then forward them on.

An unideal compromise might be to use onbeforeunload to at least prompt them to manually disconnect from the server. Many will; those that don't you can pick up with server-side detection.

answered Jul 31 '12 at 09:11

Mitya

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I do like the idea, that's better than nothing, but that still requires a user action and we can't assume that everybody will do it. – Cydonia7 Jul 31 '12 at 09:14
I agree, I'm just saying you may not have much other choice. This is a limitation of your situation, not so much my answer :) – Mitya Jul 31 '12 at 09:15
You're right, I'll keep this as a solution if I can't find anything better. Thank you for your help ! – Cydonia7 Jul 31 '12 at 09:17

Robin Maben · Answer 2 · 2012-07-31T09:20:58.567

1

window.onbeforeunload = function () {
    socket.disconnect();
    return 'This will disconnect all connections'; //Throws a prompt to user
};

UPDATE: I hadn't seen that this won't work.

Is there any way to force this to synchronous?

Probably..

var complete = false;

socket.on('disconnect', function () {
    complete = true;
});

while(!complete){
    //do nothing
}

In my case, I do..

$.ajax({
    async: false    
});

Which works fine with onbeforeunload.

edited Jul 31 '12 at 09:20

answered Jul 31 '12 at 09:08

Robin Maben

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My question is exactly how to make it synchronous for this to work because I think there is no built-in way in socket.io to do this synchronously. – Cydonia7 Jul 31 '12 at 09:12
@Skydreamer: Is tweaking the code for `socket.io` an option? :P – Robin Maben Jul 31 '12 at 09:16
I think so, but I don't know if it's possible to modify it at runtime (because I use Heroku which will install Socket.IO as it is really) – Cydonia7 Jul 31 '12 at 09:18
@Skydreamer: Another ugly way would be to use a return statement in that callback. So that there's a prompt which which should be just enough for `disconnect()` to execute. – Robin Maben Jul 31 '12 at 09:20
I've already thought of this but I really don't like it because that worsens the user experience. – Cydonia7 Jul 31 '12 at 09:21
Yes and as @Utkanos says.. You can't know whether he clicked Ok or Cancel. – Robin Maben Jul 31 '12 at 09:22
I made another edit. Does that block it enough? I have no way of testing it. – Robin Maben Jul 31 '12 at 09:26
It does an infinite loop even if I just can't understand why. – Cydonia7 Jul 31 '12 at 09:30
Wierd. Does the `complete = true` ever execute? – Robin Maben Jul 31 '12 at 09:33
A console.log next to it doesn't show anything so I believe it is not. Quite weird. – Cydonia7 Jul 31 '12 at 09:35
1

I suspect the infinite loop never releases the execution thread for the callback to do its bit.. :( – Robin Maben Jul 31 '12 at 09:40
@Robin: Yep, that's exactly what's happening. – Felix Kling Jul 31 '12 at 10:07

score 0 · Answer 3 · edited May 23 '17 at 11:59

0

Maybe you could try using the "beforeunload" event instead of "unload", like there, but I don't think there is a way to delay the window closing by another way.

edited May 23 '17 at 11:59

Community

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answered Jul 31 '12 at 09:04

Ricola3D

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I already tried beforeunload but this doesn't change anything. – Cydonia7 Jul 31 '12 at 09:07

score 0 · Answer 4 · answered Jul 31 '12 at 12:37

It now works in my local version because I have downgraded to socket.io 0.9.4. It was a bug indeed. Now it doesn't work on my host (Heroku) that has version 0.9.4 too (I configured it).

I don't know what to do to solve this but yay, at least it works in local...

score 0 · Answer 5 · edited May 23 '17 at 11:45

0

Take a look at a similar question I had, it turns out there was a bug in socket.io from 0.9.5 to 0.9.9.

Here's a link to my question. Hope this helps.

edited May 23 '17 at 11:45

Community

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answered Aug 28 '12 at 02:20

scanales

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Force a javascript code to be synchronous

5 Answers5