После окончания функции запуска

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Callback</h2>

<p>Wait 3 seconds (3000 milliseconds) for this page to change.</p>

<h1 id="demo"></h1>

<script>
setTimeout(myFunction, 1000);

function myFunction() {
  document.getElementById("demo").innerHTML = "I love You !!";
}
</script>

</body>
</html>
Xanthous Xenomorph