JSFiddle - React, Tailwind, and code Playground

HTML

<span id="myDiv"></span>

JavaScript

// THIRD FUNCTION (changes the text)
var i = 0;
function thirdFUN(){
    i++;
    var popo = document.getElementById('myDiv');
    popo.innerText = i;
}
setInterval(thirdFUN, 500);

//---------------------------------------------------------------

// JQUERY
/*var popo = $('#myDiv');
$('#myDiv').bind("DOMSubtreeModified",function(){
    if (popo.html() == "10")
    console.log('changed');
});
*/

//---------------------------------------------------------------

// PURE JS
window.onload = function(){
    
var popo = document.querySelector('#myDiv');

var observer = new MutationObserver(function(mutations){
    mutations.forEach(function(mutation){
        console.log(mutation.type); // <- It always detects changes
        if (popo.innerHTML == "10"){
            alert('hello');
        }
    });    
});

var config = {characterData: true, subtree: true};
observer.observe(popo, config);
//observer.disconnect();

}