Why function does't work?

Question

"Undo" function doesn't work properly.
https://code.sololearn.com/W1bQLVfUch1c/?ref=app

Answer 1

I think you need to use counter-1 in the undo-function for indexing:

undo.onclick= function(){
if(last[counter-1].innerHTML!="")
last[counter-1].innerHTML="";
counter--;
}