GIF图：html
代码：jquery
<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title></title>
<script src="js/jquery-3.3.1.js"></script>
</head>
<body>
<p>1</p>
<p>2</p>
<p>3</p>
<p>4</p>
<script>
$("p").on("click",function(){
alert($(this).text()-1);
});
</script>
</body>
</html>