Why setAttribute() doesn't work here?

Question

https://code.sololearn.com/WwHbcFUU4564/?ref=app

Answer 1

The JS error is due to the script executing before the document is properly loaded. Try:

window.onload = function() {
var h1 = document.getElementsByTagName("h1")[0]
h1.setAttribute("title","good");
}

Answer 2

Your DOM is not ready at the moment.
Wrap your code with
document.addEventListener('DOMContentLoaded', function(){
//Your code
});