1var elementsArray = document.getElementsByClassName("someclassname");

1var selector = document.getElementsByClassName('class-name');

1var elements = document.getElementsByClassName(names); // or:
2var elements = rootElement.getElementsByClassName(names);

1var elements = document.querySelector(".class");

1var x = document.getElementsByClassName("example");
2var i;
3for (i = 0; i < x.length; i++) {
4  x[i].style.backgroundColor = "red";
5}

1var myElement=document.getElementById("someElementID");