Guess-Game | It is a YES-OR-NO Guess Game | Game Engine library

Ok, so I'm gonna put my edited version of your code at the bottom and do the explaining up here.

First, you needed somewhere to keep track of the letters that you had already pressed. I added an array at the top of your script section to keep everything together. This is also important because it is outside the scope of the keyup event

Second, I actually added a little quality of life change in there. You weren't checking if the button pressed was actually a letter so I fixed that by wrapping everything in an if statement and then checking for the letter codes.

Then finally all were doing is using the includes() function. That's gonna check if the letter that was pressed has been seen already. If it has we do nothing. If it hasn't then we'll push that letter into the pastLetters array so that if we see it again we don't punish the user for it. Since the pastLetters array if in the parent scope of this it's persistent and won't be overridden if there's another keydown event.

Also important to note! I added that array to your reset pieces too so that when the game gets reset, the pastLetters array also gets reset.