element.classList is of DOMTokenList type.
Is there a method to clear this list?
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.