javascript - ajax - 验证数据库中是否已存在行

Question

我有一个简单的表单，用于将电子邮件地址提交到数据库。我想首先使用 javascript 和 AJAX 检查该电子邮件地址是否已存在于表中。无论该行是否已存在于数据库中，我使用的代码始终返回 true。有什么提示吗？

JS代码

<script> 
$('form').submit(function(e) 
{
  alert('submit intercepted');
  e.preventDefault(e);
  var u = $('#email').val();
  $.ajax({
      url: './test2.php',
      type: 'POST',
      data: u,
      success: function (response) {
      //get response from your php page (what you echo or print)
        $('#status').append('<p>The email is ok to use and the row has been inserted </p><p>' + response);
        console.log('Submitted');
      },
      error: function(jqXHR, textStatus, errorThrown) {
        console.log(textStatus, errorThrown);
        alert('Email exists');
      }
    });
});
</script>

HTML 代码

<form id="form" action="./test2.php" method="post"> 
    E-mail: <input type="email" name="email" id="email" required><br>
    <span id="status">   </span>
    <input type="submit" id="btnsub">
  </form>

PHP 代码

if(isset($email)) {

  $sql_email_check = $conn->query("SELECT Email FROM test WHERE Email='$email' LIMIT 1") ;

  $email_check = $sql_email_check->num_rows;

  if ($email_check < 1) {

    //return "Available";
    return true;   
    //header('Location: /test.php');
    exit();

  } 
  else 
  {
    //return "Unavailable";
    return false;
    //header('Location: /test.php');
    exit(); 

  }
}

Answer 1

所以基本上问题是将答案从 php 代码返回到 ajax 函数。由于某种原因，返回值没有按照预期正确解释(无论如何，根据我的理解)

PHP 代码

if(isset($email)) {    
  $sql_email_check = $conn->query("SELECT Email FROM test WHERE Email='$email' LIMIT 1") ;     
  $email_check = $sql_email_check->num_rows;    

  if ($email_check == 0) {
    echo 'true';  
    return; 
    } 
  else 
  {
    echo 'false';
    return;    
  }
}

JS 代码

<script> 
$('form').submit(function(e) 
{
  alert('submit intercepted');
  e.preventDefault(e);
  var u = { 'email': $('#email').val() };
  $.ajax({
      url: 'test2.php',
      type: 'POST',
      //dataType: 'string',
      data: u,
      success: function (response) {
        //get response from your php page (what you echo or print)
        console.log(response);

        if (response === 'true') {
          $('#status').append('<p>The email is ok to use and the row has been inserted </p><p>' + response);

        } else {
          console.log('Email does exist ');

        }
      },
      error: function(jqXHR, textStatus, errorThrown) {
        console.log(textStatus, errorThrown);

      }
    });
});
</script>