How to hide a all specific Classess in Javascript extension for Chrome? [duplicate]

How to hide a all specific Classes in Javascript extension for Chrome? I have seen other questions and they do not answer the question I ask. All classes named <span class="post_name"> need to be hidden. There are always different amounts of the class on the page so I don't know how much to hide but they all need to be hidden.

tried so far:

document.getElementsByClassName("post_name").style.visibility="hidden";

and

var x = document.getElementsByClassName("span.post_name");
x.style.display = "none"