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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<script>
function myfunction() {
var t=document.getElementById("demo2").value;
if(t==""||isNaN(t)){
alert("不是数字");
}
else return;
}
</script>
</head>
<body>
<h1>我的第一个 JavaScript 程序</h1>
<p id="demo">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 type="text" id="demo2">
<button type="button" onclick="myfunction()">click</button>
</body>
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