.classList

A property holding a list of CSS classes on an element. This property is read-only - it cannot be set.

let classes = element.classList;

You can access items of the list through regular indexing, or through an iterator.

for (let i = 0; i < element.classList.length; i++) {
    let cssClass = element.classList[i];
    ...
}
for (let cssClass of element.classList) {
    ...
}

See also: .classList.add(), .classList.remove(), .classList.contains(), .classList.toggle()