JSFiddle - React, Tailwind, and code Playground

HTML

<p id="demo"></p>

JavaScript

function getLastDayOfYear(datex)
{
    var x = document.getElementById("demo");
    
    var year = datex.getFullYear();
    var month = 11;
    var day = 0; // ?????
    
    x.innerHTML = day + "-" + month + "-" + year;
}

getLastDayOfYear(new Date());