mankutila

var articles = document.querySelectorAll(".element"),
    index = Math.round(Math.random() * (articles.length + 1));
articles[index].style.background = "lime";

mankutila

var tabLinks = document.querySelectorAll('ul.tabs li a');
tabLinks[0].setAttribute('class', 'tab--active');
document.querySelectorAll('section.tab-content')[0].style.display = 'block';

for (var i = 0; i < tabLinks.length; i++) {
    tabLinks[i].onclick = function() {
        var target = this.getAttribute('href').replace('#', '');
        var sections = document.querySelectorAll('section.tab-content');

        for(var j=0; j < sections.length; j++) {