change attribute of an element javascript

1var element = document.getElementById("elemId");
2element.setAttribute("attributeName", "value");

1Element.setAttribute(name, value);

1//Fetch id from html
2var a = document.getElementById("id");
3
4//variable.setAttribute("Attributes","Properties:Values")
5a.setAttribute("style", "visibility:hidden;");
6

1
2 document.getElementsByTagName("H1")[0].setAttribute("class", "democlass");