JS获取不到身份证信息

为什么用JS获取不到信息



html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>注册title>
    <script language="javascript">
        function btc(){
            var idcard;
            idcard=document.f.idnumber.value;
            var yearbirth=parseFloat(idcard.substring(6,10));
            var monthbirth=parseFloat(idcard.substring(10,12));
            var daybirth=parseFloat(idcard.substring(12,14));
            document.f.year1.value=yearbirth;
            document.f.month1.value=monthbird;
            document.f.day1.value=daybirth;
        }
    script>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        body{
            background-size: 100% 700px;
            background-repeat: no-repeat;
            background-image: url(../imafes/登录注册背景图\(1\).jpg);
        }
        div{
            width: 400px;
            height: 500px;
            background-color: rgb(129, 168, 213);
            margin: auto;
            margin-top: 100px;
        }
        p{
            line-height: 4em;
            height: 2em;
        }
    style>
head>
<body>
    <div>
        <h1 align="center">用户注册界面h1>
        <form action="" name="f">form>
        <br>
        <br>
        <p>  用户名：<input type="text" name="user"  size="33" placeholder="请输入用户名">p>
        <p>  密码：<input type="password" name="password"  size="30" placeholder="请输入密码">p>
        <p>  姓名：<input name="username" type="text" maxlenth="10" placeholder="请输入姓名" />p>
        <p>  性别：<input name="sex" type="radio" value="男" />男   
                 <input name="sex" type="radio" value="女" />女p>
        <p>  身份证号：<input name="idnumber" type="text" maxlenth="18" placeholder="请输入身份证号"/>
        p><p>  <input name="btn" type="button" value="获取身份信息" onclick="btc()">p>
        <p>  出生日期：<input type="number" name="year1" min="2000" max="2022"  />年
            <input type="number" name="month1" min="01" max="12"  />月
            <input type="number" name="day1" min="01" max="31"  />日p>
        
        <p>  联系电话：<input name="username" type="text" maxlenth="30" placeholder="请输入电话"/>p>
        <p>  邮箱：<input name="username" type="text" maxlenth="18" placeholder="请输入邮箱"/>p>
        <p>  地址：<input name="username" type="text" maxlenth="18" placeholder="请输入地址"/>p>
        <p>            
             <a href="实验二-登录.html" target="_blank" title="注册"><input type="submit" value="注册" align="center">a>a>    
    form>
    div>
body>
html>

img

建议用jQuery或者原生document.getElenentById