how to add selected attribute in javascript 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: make select option selected javascript

document.getElementById('personlist').getElementsByTagName('option')[11].selected = 'selected'