Add style to another query

Question

How do you add and remove a class to another class or query?
Example:
I want to assign “.hidden” to button on command.
.hidden {
background-color: transparent;
color: transparent;
}

button {
color: black;
background-color: white;
}

Answer 1

element.classList.add('classname')

https://www.w3schools.com/howto/howto_js_add_class.asp

Answer 2

read the linked source.

element us the html element, xlaaaList is a method.

Answer 3

Thank you! What do you put in the element and classlist selectors?