[javascript] how to append a css class to an element by javascript?

Suppose a HTML element's id is known, so the element can be refereced using:

document.getElementById(element_id);

Does a native Javascript function exist that can be used to append a CSS class to that element?

you could use setAttribute.

Example: For adding one class:

 document.getElementById('main').setAttribute("class","classOne"); 

For multiple classes:

 document.getElementById('main').setAttribute("class", "classOne classTwo"); 

Adding class using element's classList property:

element.classList.add('my-class-name');

Removing:

element.classList.remove('my-class-name');

You should be able to set the className property of the element. You could do a += to append it.

classList is a convenient alternative to accessing an element's list of classes.. see http://developer.mozilla.org/en-US/docs/Web/API/Element.classList.

Not supported in IE < 10

When an element already has a class name defined, its influence on the element is tied to its position in the string of class names. Later classes override earlier ones, if there is a conflict.

Adding a class to an element ought to move the class name to the sharp end of the list, if it exists already.

document.addClass= function(el, css){
    var tem, C= el.className.split(/\s+/), A=[];    
    while(C.length){
        tem= C.shift();
        if(tem && tem!= css) A[A.length]= tem;
    }
    A[A.length]= css;
    return el.className= A.join(' ');   
}

addClass=(selector,classes)=>document.querySelector(selector).classList(...classes.split(' '));

This will add ONE class or MULTIPLE classes :

addClass('#myDiv','back-red'); // => Add "back-red" class to <div id="myDiv"/>
addClass('#myDiv','fa fa-car') //=>Add two classes to "div"