php ajax contact form gives me undefined index error upon submission

When i try get ehco from php to ajax, all time i get "Baddd".
Why isset($data['do_login']) all time false?!
Name button is normal..
If i delete ajax function and add in form action="demo.php", then after submit form open new window(demo.php) and write "good".
But with ajax it doesnt work :(
It say error: Undefined index: do_login in demo.php on line 6, but why?!
Help pls

	<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
	  <div class="modal-dialog" role="document">
	    <div class="modal-content">
	      <div class="modal-header">
	        <h4 class="modal-title" id="myModalLabel">Увійти</h4>
	        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
	        	<span aria-hidden="true">&times;
	        	</span>
	        </button>
	      </div>
	      <div class="modal-body">
	        <form method="POST" class="js-form" id="popupResult">
				<input type="email" name="email" class="form-control" required="" 
				 placeholder="Електронна адреса">
				<input type="password" name="password" class="form-control" required="" 
				 placeholder="Пароль">
				<label>
					<input type="checkbox" name="option1">
					 Запам'ятати мене
				</label>
				<input type="submit" value="Submit" id="do_login" name="do_login"/>
	        </form>
	      </div>
	      <div class="modal-footer">
	      	<!-- <a href="" class="mr-auto ml-auto">
	      		Не отримали листа-підтвердження?
	      	</a> -->
	      <div id="result"></div>
	      </div>
	    </div>
	  </div>
	</div> 

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$("#do_login").click( function() {
	        $.ajax({
		        type: "POST",
		        //dataType: "html",
		        data: $("#popupResult").serialize(),
		        url: 'demo.php',
		        success: function(data) {
		        	alert(data);
		        	//$("#result").text(data);
		        	//alert(data);
		            //if (data == true) $("#myModal").modal('hide');
		            //if (data == false) $("#result").text("Bad");
		            //$("#myModal").modal('hide');
		        }
		    });

	    	$("#popupResult").submit( function() {
	       		return false;    
	    	});
	    });

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<?php
	require "db.php";
	
	$data = $_POST;

	echo $_POST["do_login"];
	echo $_POST["email"];
	echo $_POST["password"];

	if(isset($data['do_login']))
	{
		$errors = array();
		$user = R::findOne('login', 'email = ?', array($data['email']));
		if($user) 
		{
			if (password_verify($data['password'], $user->password))
			{
				$_SESSION['logged_user'] = $user;
				//echo '<div style="color:green;">Easy</div>';
				echo "good";
			}
			else
			{
				$errors[] = 'Неправильно введения пароль';
			}
		}
		else
		{
			$errors[] = 'Користувача з таким логіном не існує';
		}

		if(!empty($errors))
		{
			//echo '<div style="color:red;">'.array_shift($errors).'</div>';
			echo "er";
		}
	} else echo "Baddd";
?>

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