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).