Why does getElementsByClassName return undefined?

Question

window.onload= myStuff();

function myStuff(){
var spoil =document.getElementsByClassName("spoilers");

for(var i=0; i<spoil.length;i++){
spoil[i].style.color="rgb(255,0,0)";
}
}

Answer 1

window.onload should be a function and NOT a function call

Change to this:
window.onload= myStuff;

Answer 2

It's not possible to answer without watching the DOM content (HTML)

According to MDN and what I have tried in playground getElementByClassName always returns [HTMLCollection] not undefined.

Even if no elements with specified class are found then also it returns [HTMLCollection].

When no elements are found length of Collection will will be 0.

https://developer.mozilla.org/en-US/docs/Web/API/Document/getElementsByClassName

Answer 3

https://code.sololearn.com/Wtt211CWcukT/?ref=app

Answer 4

Thank you!

Answer 5

It seems that my longer code was messing up because I assigned multiple classes to the same element.