The blur event triggers when object lose focus. You can try to run the following code to learn how to implement onblur event in JavaScript.
Example
<!DOCTYPE html>
<html>
<body>
<p>Write below:</p>
<input type = "text" onblur = "newFunc(this)">
<script>
function newFunc(a) {
a.style.background = "green";
}
</script>
</body>
</html>