Javascript 通过ajax发送POST请求时出错

我想发送一个post请求，并从php文件中获取一个回音。但我找不到我的错误，总是得到一个通知：未定义的索引

请不要用jquery

<form id="register" method="POST" action="register-action.php">

    <input type="text" name="first_name" placeholder="Onoma"><br>
    <input type="text" name="last_name" placeholder="Epitheto"><br>
    <input type="text" name="mail" placeholder="E-mail"><br>
    <input type="password" name="pass" placeholder="password"><br>

    <select name="eidikotita_id">
        <?php while($eid = $eidikotites->fetch( PDO::FETCH_ASSOC )):
            echo "<option value={$eid['id_eidikotitas']}>{$eid['titlos']}</option>";
        endwhile; ?>
    </select>

    <input type="submit" value="Register"><br>
</form>

/////////////////////////////////////////////////////////////////////////////

var mail_input = document.querySelector("#registerinput[name='mail']").value;
alert(mail_input);
document.querySelector("#register input[name='first_name']").focus();
document.querySelector("#register input[name='mail']").onblur = function() {

var request = new XMLHttpRequest();//instantiate an XMLHttpRequest object
request.open("POST", "register-action.php", true);//Specifying the Request
request.setRequestHeader("Content-Type", "text/plain");//POST requests,for example, need a “Content-Type” header
request.send(mail_input);//specify the optional request body. GET requests never have a body, so you should pass null or omit the argument.
};

<?php 
echo $_POST['mail'];
?>

只需

print\u r（）

发布数据即可查看问题所在

xhr.send（）

应该是这样的：

xhr.send（'mail='+encodeURIComponent（email））

，因为您想要

$\u POST['mail']

。您的选择器也有问题，应该是

“#注册输入[name='mail']”

document.querySelector("#register input[name='first_name']").focus();
document.querySelector("#register input[name='mail']").onblur = function() {

    var mail_input = document.querySelector("#register input[name='mail']").value;
    var request = new XMLHttpRequest();//instantiate an XMLHttpRequest object
    request.open("GET", "register-action.php?mail="+mail_input, true);//Specifying the Request
    request.setRequestHeader("Content-Type", "text/plain");//POST requests,for example, need a “Content-Type” header
    request.send(encodeURIComponent(mail_input));//specify the optional request body. GET requests never have a body, so you should pass null or omit the argument.
//////send succcess

    if (request.readyState === 4 && request.status === 200) {
        request.onreadystatechange = function(){
        console.log(request.responseText);      
        }
    }   
};