Clear element.classList

I'm not aware of a "method" in the sense of a "function" associated with classList. It has .add() and .remove() methods to add or remove individual classes, but you can clear the list in the sense of removing all classes from the element like this:

element.className = "";

var classList = element.classList;
while (classList.length > 0) {
   classList.remove(classList.item(0));
}

With ES6 and the spread operator, this is a breeze.

element.classList.remove(...element.classList);

This will spread the list items as arguments to the remove method.
Since the classList.remove method can take many arguments, they all are removed and the classList is cleared.

Even though it is readable it is not very efficient. @Fredrik Macrobond's answer is faster.
View different solutions and their test results at jsperf.