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if else if

和java一样

if (time<10)//if else if else
  {
  x="Good morning";
  }
else if (time<20)//else if 与c语言不同
  {
  x="Good day";
  }
else
  {
  x="Good evening";
  }

switch,for

<script>
var day=new Date().getDay();//switch
switch (day)
{
case 0:
  x="Today it's Sunday";
  break;
case 1:
  x="Today it's Monday";
  break;
case 2:
  x="Today it's Tuesday";
  break;
case 3:
  x="Today it's Wednesday";
  break;
case 4:
  x="Today it's Thursday";
  break;
case 5:
  x="Today it's Friday";
  break;
case 6:
  x="Today it's Saturday";
  break;
default:
  x="default"
}
var person={fname:"John",lname:"Doe",age:25};//for in
for (x in person)

{

txt=txt + person[x];

}

</script>

try

<!DOCTYPE html>//错误捕捉
<html>
<head>
<script>
var txt="";
function message()
{
try
  {
  adddlert("Welcome guest!");
  }
catch(err)
  {
  txt="本页有一个错误。\n\n";
  txt+="错误描述：" + err.message + "\n\n";
  txt+="点击确定继续。\n\n";
  alert(txt);
  }
}
</script>
</head>
<body>

<input type="button" value="查看消息" onclick="message()" />

</body>
</html>

isNaN(),throw

<!DOCTYPE html>//错误抛出
<html>
<body>
<script>

function myFunction()

{

try

{

var x=document.getElementById("demo").value;

if(x=="")    throw "值为空";

if(isNaN(x)) throw "不是数字";

if(x>10)     throw "太大";

if(x<5)      throw "太小";

}

catch(err)

{

var y=document.getElementById("mess");

y.innerHTML="错误：" + err + "。";

}

}

</script>
<h1>我的第一个 JavaScript 程序</h1>

<p>请输入 5 到 10 之间的数字：</p>

<input id="demo" type="text">

<button type="button" onclick="myFunction()">测试输入值</button>

<p id="mess"></p>
</body>

</html>
</script>

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