I think it's time to tidy up some old legacy code You still have hacks for IE8 https://github.com/socketio/engine.io-client/blob/9c79ba65f51fef44e860bbeea1626fa40c3f967c/lib/xmlhttprequest.js#L24 - nobody is using ie < 11 anymore - and ie 11 is down to less than 0.7% global usage