how to add an attribute in html through js code example

Example 1: how to add attribute to selected element in javascript

// Selecting the element
    var btn = document.getElementById("myBtn");
	
    // Setting new attributes
    btn.setAttribute("class", "click-btn");
    btn.setAttribute("disabled", "");

Example 2: set property dom javascrpt

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