<!doctype html>             
<html>                     
<head>                     
  <meta charset="UTF-8">    
  <title>第一个网页</title>
  <script>
    window.onload=function(){
        var myBtn=document.getElementById("btn");
        myBtn.onclick=function(){
            var str="Hello Javascript World! "
            alert(str)
            
        }
    }
  </script>
</head>                    
<body>                
  <button id="btn"> 测试</button>
  Hello World!        
</body>                    
</html>

第一个网页A_javascript