The Document Object Model - JavaScript: Novice to Ninja - page 194

The toggle method is a particularly useful method that will add a class if an element
doesn't have it already and remove the class if it does have it. It returns true if the class
was added and false if it was removed. For example:
swim.classList.toggle('sport'); // will remove the 'sport'
class
<< false
swimm.classList.toggle('sport'); // will add the 'sport'
class back
<< true
The contains method will check to see if an element has a particular class:
swim.classList.contains('sport');
<< true
swim.classList.toggle('sport');
<< false
swim.classList.contains('sport');
<< false
Note: Adding Classes in Old Versions of Internet
Explorer
Unfortunately, the classlist property is only available in Internet Ex-
plorer version 10 and above, so if you want to support older versions of In-
ternet Explorer, you could create a function that will add an extra class to
an element, rather than just replace the current class. The addClass func-
tion takes the element and the new class name to be added as parameters. It
uses a simple if block to check if the value of the className property
is truthy. If it is, it will append the new class to the end of the current class;
otherwise, it will simply set the new class as the element's class:
function addClass(element,newClass){
if (element.className) {