<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>
<body>
<input type="text" >
<input type="button" onclick="def();"value="登录" />
<script>
function def() {
var i = document.getElementById('#21');
alert(i.value);
}
</script>
</body>
</html>