Adding class to all childrens(in first level o hierarchy) of specified element in javascript without jquery

I want to change this from jquery to javascript: $('#id_of_element').children('div').addClass('some_class');

So far all i have is this(not working):
document.getElementById('id_of_element').getElementsByTagName('div').addClass('some_class');

I have to change all my code from jquery to javascript. Is there any site with have examples of functions in javascript next to jquery? Thanks in advance for all help :)

Try

var el = document.getElementById('elem'),
    //modern browsers IE >= 10
    classList = 'classList' in el;
for (var i = 0; i < el.children.length; i++) {
    var child = el.children[i];
    if (child.tagName == 'DIV') {
        if (classList) {
            child.classList.add('test');
        } else {
            child.className += ' test'
        }
    }
}

Demo: Fiddle