If you try decodeURIComponent("%E4") in your web browser it'll fail too - so Espruino is just attempting to implement it as per the spec.
decodeURIComponent("%E4")
@Gordon started
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
If you try
decodeURIComponent("%E4")
in your web browser it'll fail too - so Espruino is just attempting to implement it as per the spec.