解惑（2）javascript 判断用户输入文本框是否为空
举个之前写的例子
<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title>注册</title>
<meta name="viewport" content="width=device-width, initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0, user-scalable=no">
<link rel="stylesheet" href="css/basic.css"/>
<link href="css/login.css" rel="stylesheet" type="text/css">
</head>
<body>
<div class="mr-tabs" id="doc-my-tabs">
<ul class="mr-tabs-nav mr-nav mr-nav-tabs mr-nav-justify">
<li class="mr-active"><a href="">注册</a></li>
</ul>
<div class="mr-tabs-bd">
<div class="mr-tab-panel mr-active">
<form method="post">
<div class="user-email">
<label for="email"><i class="mr-icon-envelope-o"></i></label>
<input type="email" name="" id="email" placeholder="请输入邮箱账号">
</div>
<div class="user-pass">
<label for="password"><i class="mr-icon-lock"></i></label>
<input type="password" name="" id="password" placeholder="设置密码">
</div>
<div class="user-pass">
<label for="passwordRepeat"><i class="mr-icon-lock"></i></label>
<input type="password" name="" id="passwordRepeat" placeholder="确认密码">
</div>
<div class="user-pass">
<label for="passwordRepeat"><i class="mr-icon-mobile"></i><span style="color:red;margin-left:5px">*</span></label>
<input type="text" name="" id="tel" placeholder="请输入手机号">
</div>
</form>
<div class="login-links">
<label for="reader-me">
<input id="reader-me" type="checkbox"> 点击表示您同意商城《服务协议》
</label>
<a href="login.html" class="mr-fr">登录</a>
</div>
<div class="mr-cf">
<input type="submit" name="" onclick="mr_verify()" value="注册" class="mr-btn mr-btn-primary mr-btn-sm mr-fl">
</div>
</div>
</div>
</div>
</body>
<script>
function mr_verify(){
//获取表单对象
var email=document.getElementById("email");
var password=document.getElementById("password");
var passwordRepeat=document.getElementById("passwordRepeat");
var tel=document.getElementById("tel");
//验证项目是否为空
if(email.value==='' || email.value===null){
alert("邮箱不能为空！");
return;
}
if(password.value==='' || password.value===null){
alert("密码不能为空！");
return;
}
if(passwordRepeat.value==='' || passwordRepeat.value===null){
alert("确认密码不能为空！");
return;
}
if(tel.value==='' || tel.value===null){
alert("手机号码不能为空！");
return;
}
if(password.value!==passwordRepeat.value ){
alert("密码设置前后不一致！");
return;
}
//验证邮件格式
apos = email.value.indexOf("@")
dotpos = email.value.lastIndexOf(".")
if (apos < 1 || dotpos - apos < 2) {
alert("邮箱格式错误！");
}
else {
alert("邮箱格式正确！");
}
//验证手机号格式
if(isNaN(tel.value)){
alert("手机号请输入数字！");
return;
}
if(tel.value.length!==11){
alert("手机号是11个数字！");
return;
}
alert('注册成功！');
}
</script>
</html>