The onfocus event is triggered when element gets focus. You can try to run the following code to learn how to implement onfocus event in JavaScript.
Example
<!DOCTYPE html>
<html>
<body>
<p>Write below:</p>
<input type = "text" onfocus = "newFunc(this)">
<script>
function newFunc(x) {
x.style.background = "blue";
}
</script>
</body>
</html>