Answer 1

Try to add:
if(username.indexOf(" ")!==-1) {
alert("Username must not contain spacing.");
return false;
}

https://code.sololearn.com/WWK6ulX3vhdX/?ref=app

Answer 2

Calviղ Very brilliant soln

Answer 3

Try this:
if(username.match(/\s/) != null){
alert("")
return false;
}