验证用户名，要求 1、不能为空 2、不能小于6位数大于20位数 3、首字母不能大写
<body>
<input type="text" id="username" />
<p id="info"></p>
</body>
<script>
var username = document.getElementById("username");
var info = document.getElementById("info");
username.onblur = function(){
info.innerText = "";
if(!username.value){
info.innerText = "用户名不能为空";
}
if((username.value.length>0 && username.value.length<6) || username.value.length>20){
info.innerText = "用户名不合法";
}
if(username.value.charCodeAt(0)>=65 && username.value.charCodeAt(0)<=90){
info.innerText = "首字母不能大写";
}
}
</script>
