demo.html

<html>
    <head>
        <title>exists.js</title>
    </head>
    <body>
        <div id="demo">
            <div class="element1"></div>
        </div>
        <script src="https://cdn.jsdelivr.net/gh/mathdevelop/exists.js@latest/exists.min.js"></script>
        <script>
            setTimeout(function() {
				document.querySelector('#demo').insertAdjacentHTML('beforeend', '<div class="element2"></div>');
			}, 5000);
			
			exists('.element2').then(function() {
				console.log('Element found after 5 seconds');
				
				setTimeout(function() {
					document.querySelector('#demo .element1').insertAdjacentHTML('beforeend', '<div class="element3"></div>');
				}, 5000);
			});
			
			exists('.element3', '.element1').then(function() {
				console.log('Element found after 10 seconds');
			});
        </script>
    </body>
</html>