$(selector).load(URL,data,callback);
callback回调函数的参数
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<script src="https://cdn.bootcss.com/jquery/3.3.1/jquery.js"></script>
<title>load方法</title>
</head>
<body>
<p></p>
<script type="text/javascript">
$("p").load("demo.txt", function () { alert("success"); });
</script>
</body>
</html>