what is the problem here ? The code run on code pen without any errors!!

Question

https://code.sololearn.com/W0Gx3XT4aBQ2/?ref=app

Answer 1

Just wrap your JS script inside...
window.onload = function() {
// code
}

Answer 2

u can do it also by add onclick and give a name to function

Answer 3

https://code.sololearn.com/WsE97336pJri/?ref=app

Answer 4

uncaught type error: cannot set property 'onclick' of null
line: 4