Question

I want to know if there is a way to getElementByClassName("classname").innerHTML function or something to the equivalent of getElementById("ClassName").innerHTML.

Answer 1

You are missing an s in your function name. getElementsByTagName returns a collection of elements, of elements, which you need to iterate over:

var elements = document.getElementsByClassName("classname");

for (var i = 0; i < elements.length; i++) {
    elements[i].innerHTML = 'foo';
}

IE8 and below don’t support getElementsByClassName, so you’ll have to find a polyfill or use querySelectorAll (IE8).