1、getElementById() 方法:
getElementById() 方法可返回对拥有指定 ID 的第一个对象的引用。
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>获取元素</title>
<script>
window.onload=function(){
var uname=document.getElementById("myname");
alert(uname);
}
</script>
</head>
<body>
<form action="new_file.html" method="post">
用户名:<input type="text" name="myname" size="12" id="myname"><br>
密 码:<input type="password" name="password" size="6" ><br>
</body>
</html>

获取值:
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>获取元素</title>
<script>
window.onload=function(){
var uname=document.getElementById("myname");
var uValue=uname.value;
alert(uValue);
}
</script>
</head>
<body>
<form action="new_file.html" method="post">
用户名:<input type="text" name="myname" size="12" id="myname"><br>
密 码:<input type="password" name="password" size="6" ><br>
</body>
</html>
来源:https://www.cnblogs.com/zhai1997/p/12215328.html