<!DOCTYPE html>
<html>
<head>
<title></title>
<script type="text/javascript" src="jquery-3.3.1.min.js"></script>
</head>
<body>
<script type="text/javascript">
$(document).ready(function() {
$str="我是一个声明变量"
alert($str);
})
</script>
</body>
</html>