We have published a minor version of the client: 2.3.1
debug dependency has been reverted to
~3.1.0, as the newer versions contains ES6 syntax which breaks in IE browsers.
Please note that this only applied to users that bundle the Socket.IO client in their application, with webpack for example, as the "official" bundles (in the dist/ folder) were already transpiled with babel.
For webpack users, you can also take a look at the webpack-remove-debug plugin.
Get the latest client from the CDN:
For an explanation between the different bundles, please see here.