Help needed on my JavaScript

Question

I was codig this on an code app complier and when i preview it on the app it work but when i copy it to sololearn and try to pre view it says "on line 9 addEventlostener for null', the JavaScript have a bug so if u have any advice...
https://code.sololearn.com/W8N85ueyBf4E/?ref=app

Answer 1

Dagota

Write whole JS code inside this function:

window.onload = function () {

}