The fact that you’ve done this in a Code Component in the Python, means that you’d need a similar facility in JavsScript. I don’t know it off the top of my head but I think something in Pixi will work (that’s the library that we use for the stimulus rendering in JS).
From a google it looks like you need to create a function that runs on a uniCharCode event:
https://www.w3schools.com/jsref/event_key_which.asp
I dare say we will add functions to PsychoPy/PsychoJS to make this easier but, right now, in both cases it is a matter of custom code right now.