How to get someone’s Computer Username in JavaScript. Like the “whoami” Command in CMD

I’m trying to make a meta game and for one of the planned scenes I want the player’s name inside a book. The thing is… I’m using PenguinMod, a block programming language that has almost nothing to do with JavaScript. Except for an Extension that you can install with blocks that run diffrent JavaScript commands depending on what you put in them. Example Image.The Javascript Blocks in Question

So I tried looking up tutorials and found nothing. I even tried looking on here [Stack Overflow] and found nothing. I was expecting to find a command you could input that’ll grab the Name you use on your computer but it just doesn’t seem like that exists. The only thing I could get from what I lookuped was how to get a Users IP Adress. So now my only hope is that you can help out in some way. Thank you.