3.1.2 gives client error on ESM but not when loading from CDN

[Leitet]

We have run into an issue that feels like a bug.

When upgrading from 3.1.1 to 3.1.2 (npm-package) we get a client error:

This happens when loading the client module using ESM imports import 'socket.io.js'.
Loading the same 3.1.2 using a script tag (from /socket.io/socket.io.js or CDN) does not produce this error.
Loading 3.1.1 with ESM import does not produce the error.

(We are using Socket.io in a university course and the students started reporting problems last week)

[Leitet]

Changing from 'window' to 'this' (this is undefined) is probably what introduced the error:

socket.io/client-dist/socket.io.js

Line 15 in 225ade0

})(this, function() {

Loading a script using ESM will not set "this" to "window". This is undefined in a ESM-module. Loading the script using "script-tags" will set this to window though.

[Leitet]

It should probably be: "self"

[darrachequesne]

I could indeed reproduce, thanks. We will publish a fix as soon as possible.

[darrachequesne]

This should be fixed in socket.io-client@3.1.3 and socket.io-client@4.0.0, could you please check?

[Leitet]

So after som research, it boils down to globalObject in webpack.config:

  output: {
    filename: "socket.io.js",
    library: "io",
    libraryTarget: "umd",
    globalObject: "this",
  },

• self is default and should work in every "browser case", workers, ESM and old school clientJS.
• window would work in ESM and osc-JS but not in workers or Node.js.
• this will work in osc-JS, workers and Node.js but not using ESM in client.

[Leitet]

Removing or changing to 'self' should remove the problem.