Where is the Socket.IO client-side .js file located?

If you did npm install then the client socket.io file is located at node_modules/socket.io-client/dist/socket.io.js

Source: Socket get-started page

The client is available in a few ways:

• supplied by the socket.io server at /socket.io/socket.io.js
• included via webpack as the module socket.io-client
• via the official CDN https://cdnjs.cloudflare.com/ajax/libs/socket.io/<version>/socket.io.js

For the first one, the server can be configured in a couple of ways:

// standalone
var io = require('socket.io')(port);

// with existing server from e.g. http.createServer or app.listen
var io = require('socket.io')(server);

I would suggest checking if your node_modules directory is at the top level of your app directory. Also, I do believe you need to specify a port number; you should write something like var socket = io.connect('http://localhost:1337');, where the port number is 1337.

I managed to eventually answer this for myself.

The socket.io getting started page isn't clear on this, but I found that the server side of socket.io automatically hosts the .js file on starting node, in the directory specified in the documentation:

"/socket.io/socket.io.js"

So you literally just point to this url regardless of your web app structure, and it works.