js get element by id and amend class 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");