Why JS doesn't work here?

Question

But it works fine in codepen for example.. https://code.sololearn.com/WLvPy3EQkDVK/#js

Answer 1

when you write web codes on sololearn your js should be kickstarted with window.onload

Answer 2

The reason why is that sololearn puts your javascript codes in the head element but codepen puts it in the end of your document.