javascript - TypeError: listener must be a function - Stack Overflow

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I'm storing an event listener inside a WeakMap and upon retrieving the function to remove it from the EventEmitter this error get's thrown:

TypeError: listener must be a function

I just can't figure out what the problem is:

var EventEmitter = require('events').EventEmitter;

var listeners = new WeakMap();
var eventEmitterA = new EventEmitter();
var eventEmitterB = new EventEmitter();

eventEmitterA.on('open', (obj) => {
  function stuff() { /* ... */ }

  listeners.set(obj, stuff);

  eventEmitterB.on('stuff', stuff);
});

eventEmitterA.on('close', (obj) => {
  var listener = listeners.get(obj);

  console.log(typeof listener); // logs "function"

  eventEmitterB.removeListener(listener);
});


var someObject = {some: 'object'};

eventEmitterA.emit('open', someObject);
eventEmitterA.emit('close', someObject);

I'm storing an event listener inside a WeakMap and upon retrieving the function to remove it from the EventEmitter this error get's thrown:

TypeError: listener must be a function

I just can't figure out what the problem is:

var EventEmitter = require('events').EventEmitter;

var listeners = new WeakMap();
var eventEmitterA = new EventEmitter();
var eventEmitterB = new EventEmitter();

eventEmitterA.on('open', (obj) => {
  function stuff() { /* ... */ }

  listeners.set(obj, stuff);

  eventEmitterB.on('stuff', stuff);
});

eventEmitterA.on('close', (obj) => {
  var listener = listeners.get(obj);

  console.log(typeof listener); // logs "function"

  eventEmitterB.removeListener(listener);
});


var someObject = {some: 'object'};

eventEmitterA.emit('open', someObject);
eventEmitterA.emit('close', someObject);

• javascript
• node.js
• eventemitter

Share Improve this question Follow edited Sep 11, 2015 at 22:35 Bergi 666k161161 gold badges1k1k silver badges1.5k1.5k bronze badges asked Sep 10, 2015 at 21:59 OlivenbaumOlivenbaum 1,00611 gold badge77 silver badges1717 bronze badges 3

• What is typeof listener in eventEmitterA.on('close', ...)? You have a console.log() if the typeof is function, but what is it actually there? – jfriend00 Commented Sep 10, 2015 at 22:06
• console.log(typeof listener) yields "function" – Olivenbaum Commented Sep 10, 2015 at 22:07
• 1 some docs might be helpful – Hacketo Commented Sep 10, 2015 at 22:08

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1 Answer 1

Sorted by: Reset to default 3

eventEmitterB.removeListener(listener);

should be

eventEmitterB.removeListener('stuff', listener);

removeListener takes two arguments and the function is supposed to be the second argument, so you are passing the function as the name of the handler and undefined as the handler.

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