Vic Mercier3,276 Points
Steven Parker179,855 Points
There's a few different ways to do this.
If I understand correctly, you want to add a class to an HTML element. One way would be to set the className value of that element. Another would be to call the classList.add() method. Here's some examples:
myElement.className = "special" // this makes "special" the only class myElement.classList.add("special") // this adds the class but leaves any already there