当元素在HTML中获得焦点时，请使用onfocus属性执行脚本。
您可以尝试运行以下代码以实现onfocus属性-
<!DOCTYPE html>
<html>
<body>
<p>Enter subject name below,</p>
Subject: <input type = "text" id = "sname" onfocus = "display(this.id)">
<br>
<script>
function display(val) {
document.getElementById(val).style.background = "blue";
}
</script>
</body>
</html>