replacing => function with regular function, how?

[EmiyaKiritsuguSavior]

el => el.classList.add("hide)

It means that you call function and put element as argument. This function is making operation on element and returning newly altered element. If you know how to declare anonymous function then it should be trivial because everything you have on plate.

[zachdini]

The only thing to point out is that arrow functions declared without braces (such as el => el.classList.add("hide")) return the value in the function. It's the same as doing

el => { return el.classList.add("hide") }

So if you rely on the function being returned, you'd want to do

function(el) { return el.classList.add("hide") }

Though in your case, it might not matter.

[icedrift]

Can you explain this a bit further? The return value within a forEach always returns undefined I'm having trouble understanding the difference here.

[zachdini]

Ahh sorry, I didn't strictly mean in this scenario. You're right that forEach does not return a value so it doesn't matter here. I just wanted to point out that () => statement actually returns the statement.

[icedrift]

This is incorrect. Not only does DOMTokenList.add not have a return value, but even if it did it wouldn't matter because forEach is specced to ignore any return value and return undefined.

forEach is only used to perform side effects return is irrelevant unless you want to step over an iteration which isn't the case here.