The onfocusin event triggers when an element is to lose focus. You can try to run the following code to learn how to implement onfocusout event in JavaScript.
Example
<!DOCTYPE html>
<html>
<body>
<p>Write below:</p>
<p>After losing focus, the background color of input check will change.</p>
<input type = "text" onfocusout = "newFunc(this)">
<script>
function newFunc(x) {
x.style.background = "blue";
}
</script>
</body>
</html>