try：语句测试代码块的错误，当try中的代码块出错时执行catch中的代码块。
catch：语句处理错误；
throw：语句创建或抛出自定义异常。
三者一起使用可以控制程序流并生成自定义异常信息。
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>just test</title>
<script>
var txt = "";
function message()
{
try
{
adddlert("welcome guest!");
}
catch(e)
{
txt += "本页有一个错误\n\n";
txt += "错误描述：" + e.message + "\n\n";
txt += "点击确定继续。";
alert(txt);
}
}
</script>
</head>
<body>
<button type="button" onclick="message()">查看消息</button>
</body>
</html>
异常可以是字符串、数字、逻辑值、或对象
1 <script> 2 function myFunction() 3 { 4 try 5 { 6 var x=document.getElementById("demo").value; 7 if(x=="") throw "empty"; 8 if(isNaN(x)) throw "not a number"; 9 if(x>10) throw "too high"; 10 if(x<5) throw "too low"; 11 } 12 catch(err) 13 { 14 var y=document.getElementById("mess"); 15 y.innerHTML="Error: " + err + "."; 16 } 17 } 18 </script> 19 20 <h1>My First JavaScript</h1> 21 <p>Please input a number between 5 and 10:</p> 22 <input id="demo" type="text"> 23 <button type="button" onclick="myFunction()">Test Input</button> 24 <p id="mess"></p>