概括、在WEB中一共有四种方式可以引用js代码。
1、<script>直接引用代码</script>
2、<script src=xxx.js></script> //间接引用代码
3、<script defer>间接延迟引用代码</script>
4、javascipt:在地址栏中直接执行代码
示例：
<html>
<head>
<!--first run-->
<script>
alert("embedded script is in the head!");
</script>
<!--second run-->
<script src=out.js></script>
<script defer>alert("delay script in the head!");</script>
<!--在IE下是加载完body才运行，在firefox下是加载完head就运行-->
</head>
<body>
<!--third run-->
<script>
alert("embedded script is in the body!");
</script>
<!--fourth run-->
<script src=out.js></script>
<input type="button" value="button1" onclick='alert("button1 is clicked!")'/>
<!--所有的事件都以on开头-->
</body>
</html>
<!--还可以在地址栏中输入JavaScript代码，javascript:alert("script in the url!");-->
out.js
alert("out script!");
来源：https://www.cnblogs.com/eroshn/archive/2011/06/24/2088651.html