<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html;charset=UTF-8">
<title>Document</title>
<script>
window.onload=function() {
var txt = document.getElementById("txt");
var btn = document.getElementById("btn");
btn.onclick = function () {
//alert(typeof txt.value); //错误写法：alert(typeof txt) ,另外input取过来的都是字符型
// alert(Number(txt.value)); //利用Number可以转成数值型
//alert(Number(txt.value)+Number(txt.value));//转成数值型可以相加
alert(txt.value+txt.value); //会直接字符相连
}
}
</script>
</head>
<body>
<input type="text" id="txt"/>
<button id="btn">点击</button>
</body>
</html>