Hii, you could try with a coin flip:

else if (racc.checked) {Math.random() > 0.5 ? "and wearing [randAcc]" : ""}

Math.random() is uniformly distributed, so there's roughly a 50% chance that the result is above 0.5, and thus a 50% chance that you get an accessory.

For different odds, play around with adjusting the threshold (0.5). Make it higher to make accessories less likely, decrease it to make it more likely.

EDIT: Oh, right, I forgot -- you can also calculate the threshold based on how many accessories you have. It should go something like this:

Math.random() > 1/accessories.getLength

If you have five elements in accessories, 1/5 gives you 0.2, so the odds would be about 80% of the time you'll get an accessory. So that's another thingy to play around with.

EDIT2: Just correcting my own faulty math :D

There are many ways, one simple is to use .replace("and wearing random", ""), so when the random is selected, "wearing random" will be replaced by an empty string "" (nothing). you put it in wAcc in the prompt, like: wAcc.replace("and wearing random", "")