tryjs_intro_validate 在线代码实例

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<!DOCTYPE html>
<html>
<head> 
<meta charset="utf-8"> 
<title>W3Cschool教程(w3cschool.cn)</title> 
</head>
<body>
    
<h1>我的第一段 JavaScript</h1>
<p>请输入数字。如果输入值不是数字，浏览器会弹出提示框。</p>
<input id="demo" type="text">
<script>
function myFunction()
{
    var x=document.getElementById("demo").value;
    if(x==""||isNaN(x))
    {
        alert("不是数字");
    }
}
</script>
<button type="button" onclick="myFunction()">点击这里</button>
    
</body>
</html>
 
 
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运行结果
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