get element by id update existing class code example

Example 1: javascript change element class

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

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

Example 2: change elements class javascript

document.getElementById("MyElement").classList.add('MyClass');

document.getElementById("MyElement").classList.remove('MyClass');

if ( document.getElementById("MyElement").classList.contains('MyClass') )
{
document.getElementById("MyElement").classList.toggle('MyClass');
}