how to add attribute using dom javascript code example

Example 1: javascript setattribute

element.setAttribute(<name>, <value>);
// ex
var aElement = document.querySelector("a"); 
aElement.setAttribute("href", "https://isitchristmas.com/"); 
// same as <a href="https://isitchristmas.com/"></a>

Example 2: set property dom javascrpt

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