Using emit throw error: Uncaught TypeError: Cannot read property 'apply' of undefined

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Current behaviour

What is actually happening? Using emit throws error: Uncaught TypeError: Cannot read property ‘apply’ of undefined.

Steps to reproduce (if the current behaviour is a bug)

First, import Socket.io-client and use:

import io from 'socket.io-client';
const socket = io('http://localhost:3000');

Second, try emit:

socket.emit(`${this.evPrefix}::sync_upload_settings`);

It throws this error:

index.js:133 Uncaught TypeError: Cannot read property 'apply' of undefined
    at Socket.push../node_modules/component-emitter/index.js.Emitter.emit (index.js:133)
    at Socket.push../node_modules/socket.io-client/lib/socket.js.Socket.emit (socket.js:138)
    at Socket.push../node_modules/socket.io-client/lib/socket.js.Socket.onconnect (socket.js:335)
    at Socket.push../node_modules/socket.io-client/lib/socket.js.Socket.onpacket (socket.js:232)
    at Manager.<anonymous> (index.js:21)
    at Manager.push../node_modules/component-emitter/index.js.Emitter.emit (index.js:133)
    at Manager.push../node_modules/socket.io-client/lib/manager.js.Manager.ondecoded (manager.js:345)
    at Decoder.<anonymous> (index.js:21)
    at Decoder.push../node_modules/component-emitter/index.js.Emitter.emit (index.js:133)
    at Decoder.push../node_modules/socket.io-parser/index.js.Decoder.add (index.js:251)

It actually coming from here(source from socket.io.dev.js):

Emitter.prototype.emit = function(event){
	this._callbacks = this._callbacks || {};
	var args = [].slice.call(arguments, 1)
		, callbacks = this._callbacks['$' + event];

	if (callbacks) {
		callbacks = callbacks.slice(0);
		for (var i = 0, len = callbacks.length; i < len; ++i) {
			callbacks[i].apply(this, args);    // ------> HERE!!!!
		}
	}

	return this;
};

Expected behaviour

What is expected? I want to send event to server, but it doesn’t.

Setup

• OS: macOS 10.13.1
• browser: Chrome 72.0.3626.121
• socket.io version: 2.2.0
• socket.io.client version: 2.2.0
• node version: 11.0.0

Other information (e.g. stacktraces, related issues, suggestions how to fix)

I used Webpack for bundling.

[WaylayOvercast]

I actually was stuck on this, although it may be for a different reason… I had run into this issue trying to create my own form of “authentication” (if you can call it that.) - and I found my issue…although I don’t understand it. see below. So I was returning a listener for the servers response and checking in an if statement to see if the return of that function had the “.connected” attribute… yes I know quite hacky but… my problem came up later when I had tried to return “socket.on(“allgood”)” without the empty anon function… basically returning socket.on without its second argument caused it for me…

not sure this is helpful for anyone as if you use the library correctly it is very well put together and I definitely don’t suggest doing what I did here lol

[Osmiogrzesznik]

For Anybody Searching for this error. I had similar problem. It can be caused by providing variable as a second parameter in .on method that is undefined. Internal code of socket.io tries to call .apply method to pass arguments into your event handler. If function registered is not existing and therefore undefined, this error occurs since undefined values have no such property . It often may be the case of the typo in the name of the function

i.e

socket.on('next_player', this.nextplayer)

should be :

socket.on('next_player', this.next_player)

make sure that

${this.evPrefix}::sync_upload_settings

exists in the moment when the event happens