How to properly remove event listeners in node js eventemitter

Question

I have a set up with a GET and a POST route, the idea being that POSTing to the route triggers an event, and the GET route is a server-sent eventstream which fires each time the POSTed event is triggered... however, i think i've done something wrong as event listeners seem to get added routinely despite only having one event stream subscriber... what am i doing wrong?

var events = require('events'),
EventEmitter = events.EventEmitter,
rr = new EventEmitter();

app.post('/api/:boardname/remoterefresh', function(req, res){
    var boardname = req.param('boardname'),
    data = new Date().getTime();
    rr.emit("refresh-"+boardname, data)
    res.json({data: data})
});

app.get('/api/:boardname/remoterefresh', function(req, res){
    var boardname = req.param('boardname')

    rr.on("refresh-"+boardname, function(data){
        setTimeout(function(){
            res.write('data: '+data+'\n\n');
        }, 1000)
    });

    req.socket.setTimeout(Infinity);

    res.writeHead(200, {
        'Content-Type': 'text/event-stream',
        'Cache-Control': 'no-cache',
        'Connection': 'keep-alive'
    });

    res.write('\n');

    req.on('close', function(){
        console.log('closed')
        rr.removeListener("refresh-"+boardname, function(){
            //
        })
    })

})

Selfcontrol

possible duplicate of Unbinding events in Node.js – Smyth

Could this cause a race condition and leak data across requests? – Corelative

Answer 1

You should name the function you attach as event handler. Then on removing it, you just pass the function by name:

app.get('/api/:boardname/remoterefresh', function(req, res){
    var boardname = req.param('boardname')
    function refreshHandler(data){
        setTimeout(function(){
            res.write('data: '+data+'\n\n');
        }, 1000)
    }
    rr.on("refresh-"+boardname, refreshHandler);

    req.socket.setTimeout(Infinity);

    res.writeHead(200, {
        'Content-Type': 'text/event-stream',
        'Cache-Control': 'no-cache',
        'Connection': 'keep-alive'
    });

    res.write('\n');

    req.on('close', function(){
        console.log('closed')
        rr.removeListener("refresh-"+boardname, refreshHandler);
    });
});

Basically removeListener will look up the given function by reference, if it found that function it will remove it from the event hander.

Smyth

thanks man. you are the life saver :) it was joke. but I found solution of my problem – Chamaeleon

Answer 2

To avoid referencing the anonymous listener callback function, you can dangerously wipe registered listeners with the emitter.removeAllListeners method:

rr.removeAllListeners("refresh-"+boardname)

Be aware that this approach can easily result in unintended side-effects (deregistering listeners registered elsewhere in the codebase), and should be reserved for "wipe the slate clean" use cases (eg: Testing)

Demonstrator

note that rr.removeAllListeners (in the plural) is the correct name – Emmerich

I also found I needed to use removeAllListeners, when removeListener didn't seem to recognise a method name inside a class which was also bound with function.bind(this), so perhaps that doesn't work. – Stumer