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(){             //         })     })  }) 

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.

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)