Discussion on: A Beginners Guide To DOM Manipulation

document.getElementByClass("className")
//This method only returns the child element of the specified Class.

document.getElementsByClassName("className")
//This method returns all elements inside the whole document by the class you specified.

document.getElementById("someElement").getElementsByClassName("className")
//It works in all elements of the DOM, this will return all elements by the class you specify inside the element you want