html代码：
<!doctype html public "-//w3c//dtd xhtml 1.0 transitional//en" "http://www.w3c.org/tr/xhtml1/dtd/xhtml1-transitional.dtd">
<html xmlns="http://www.w3x.org/1999/xhtml">
<head>
<title>Javascript Day01</title>
<meta http-equlv="content-type" content="text/html;charset=utf-8"/>
<script language="javascript" src="js_demo1.js" type="text/javascript"></script>
<script language="javascript" type="text/javascript">
function firstMethod(){
alert("Hello World in script block.");
}
</script>
</head>
<form>
<h2>1.Hello Word</h2>
<input type="button" value="first button" onclick="alert('Hello Word');"/>
<input type="button" value="second button" onclick="firstMethod();"/>
<input type="button" value="third button" onclick="secondMethod();"/>
</form>
</body>
</html>
js_demo1.js
function secondMethod(){
alert("hello world");
}