get the class and change js code example

Example 1: js add class

var element = document.getElementById('element');
element.classList.add('class-1');
element.classList.add('class-2', 'class-3');
element.classList.remove('class-3');

Example 2: javascript change element class

//Overwrite all classes
element.className = "class";

//Add class
element.classList.add("class");