var element = document.getElementById('element');
element.classList.add('class-1');
element.classList.add('class-2', 'class-3');
element.classList.remove('class-3');document.getElementById("myElementID").classList.add('myClassName');
document.getElementById("myElementID").classList.remove('myClassName');//Overwrite all classes
element.className = "class";
//Add class
element.classList.add("class");function switchMode(){
// Body switching
if(document.getElementById("page-body").className == "page-body-day"){
document.getElementById("page-body").className = "page-body-night";
}else{
document.getElementById("page-body").className = "page-body-day";
}
// Heading switching
if(document.getElementById("main-heading").className == "main-heading-day"){
document.getElementById("main-heading").className = "main-heading-night";
}else{
document.getElementById("main-heading").className = "main-heading-day";
}
}