Uncaught ReferenceError: Cannot access 'XMLHttpRequest' before initialization

You want to:

• report a bug
• request a feature

Current behaviour

When importing the client in a React or Svelte project I’m getting the Uncaught ReferenceError: Cannot access 'XMLHttpRequest' before initialization error.

Issue is coming from engine.io-client:

const hasXHR2 = (function() {
  const XMLHttpRequest = require("xmlhttprequest-ssl"); //<---- Devtools says the problem is here
  const xhr = new XMLHttpRequest({ xdomain: false });
  return null != xhr.responseType;
})();

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

I can’t provide a REPL because I keep getting: Could not find module in path: 'xmlhttprequest-ssl' relative to '/node_modules/engine.io-client/lib/transports/index.js' which somehow seems to be related to the issue described in Current behaviour.

The Uncaught ReferenceError: Cannot access 'XMLHttpRequest' before initialization will show up by simply importing the package like so: import io from 'socket.io-client.

It doesn’t matter what code you have after that line, it’ll crash with the error.

Expected behaviour

I’m expecting to be able to import the package.

Setup

• OS: macOS & Windows give the issue
• browser: Edge, Chrome, Firefox and Safari give the issue.
• socket.io version: Client is 3.0.3 - Server is N/A

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

polling-xhr.js:18 Uncaught ReferenceError: Cannot access 'XMLHttpRequest' before initialization
    at polling-xhr.js:18
    at polling-xhr.js:21
    at main.ts:5

[takoyaro]

socket.io-client/dist/socket.io.min.js and socket.io-client/dist/socket.io.msgpack.min.js works I guess 🤔

It does “work” in the sense that the error disappears but it does not work as Client<=>Server communication never establishes. Simply importing Socket.io v2 “fixes” my issue…

[darrachequesne]

This should be fixed by https://github.com/socketio/engine.io-client/commit/c22681542cc4b0acf2dc5ac2923fa3faa33b6008 (included in engine.io-client@4.0.5).