阻止form空表单提交----JavaScript

<div class="warp">
        <h2>登录到pfan空间</h2>
        <p>这里有更多的知识分享，交流。</p>
        <form action="/register" method = "post" >
            <input type="text" name = "username" placeholder = "请输入注册用户名">  <br />
            <input type="password" name = "pwd" placeholder = "请输入初始密码">   <br />
            <input type="password" name = "aginpwd" placeholder = "请再次输入密码">   <br />
            <input type="submit" value ="注册" class="login_btn">
        </form>
    </div>
<script>
    var form = document.forms[0],
        submit = document.querySelector(".login_btn"),
        inputBtn = document.getElementsByTagName("input");
        console.log(form);
    form.onsubmit = function(){
        if(inputBtn[0].value == ""){
            alert("请您认真填写注册用户名！");
            return false;
        }else if(inputBtn[1].value == ""){
            alert("请您认真输入初始密码！");
            return false;
        }else if(inputBtn[2].value == ""){
            alert("请您再次输入密码");
            return false;
        }else if(inputBtn[1].value != inputBtn[2].value){
            alert("两次密码输入不匹配，请更正！");
            return false;
        }    
        
    }
</script>

<div class="warp">
        <h2>登录到pfan空间</h2>
        <p>这里有更多的知识分享，交流。</p>
        <form action="/register" method = "post" >
            <input type="text" name = "username" placeholder = "请输入注册用户名">  <br />
            <input type="password" name = "pwd" placeholder = "请输入初始密码">   <br />
            <input type="password" name = "aginpwd" placeholder = "请再次输入密码">   <br />
            <input type="submit" value ="注册" class="login_btn">
        </form>
    </div>
<script>
    var form = document.forms[0],
        submit = document.querySelector(".login_btn"),
        inputBtn = document.getElementsByTagName("input");
        console.log(form);
    form.addEventListener("submit",function(e){
        if(inputBtn[0].value == ""){
            alert("请您认真填写注册用户名！");
            e.preventDefault();
        }else if(inputBtn[1].value == ""){
            alert("请您认真输入初始密码！");
            e.preventDefault();
        }else if(inputBtn[2].value == ""){
            alert("请您再次输入密码");
            e.preventDefault();
        }else if(inputBtn[1].value != inputBtn[2].value){
            alert("两次密码输入不匹配，请更正！");
            e.preventDefault();
        }    
    },false);

</script>