代码如果我在单独的文件中运行相同的代码，它会运行。
$("#datepicker").datepicker();<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
<input type="text" id="datepicker">看起来你错过了JQuery Datepicker的主题css库:
<link rel="stylesheet" href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">
<link rel="stylesheet" href="/resources/demos/style.css">
请看看运行良好的样品:
<!doctype html>
<html lang="en">
<head>
<title>jQuery UI Datepicker- Solution</title>
<link rel="stylesheet" href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">
<link rel="stylesheet" href="/resources/demos/style.css">
<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
<script>
$( function() {
$( "#datepicker" ).datepicker();
});
</script>
</head>
<body>
<p>Date: <input type="text" id="datepicker"></p>
</body>
</html>