PHP Ajax警报将无法正常工作

Question

I am trying to check user is exist from data using PHP and Ajax. Using the following codes:

Ajax

$('#btn_check_pc').click(function() {
  username = $("#username").val();
  $.ajax({
    type: 'POST',
    url: 'process.php',
    data: "username=" + username,
    dataType: 'json',
    beforeSend: function() { //Do Something    
    },
    success: function(response) {
      if (response == 'used') {
        console.log("Username Already In Use");
      }
    }
  });
});

PHP

<?php
    include_once "includes/get_data.php" ;

    $username = $_POST["username"];
    //Validating purchase

    $checkUserRegistered = mysqli_query($db,"SELECT  * FROM  users WHERE username = '$username'") or die(mysqli_error($db));

    if(!mysqli_num_rows($checkUserRegistered)){
        // Do Something
    }else{
         echo 'used';
    }
?>

So When I click the #btn_check_pc button then the PHP code will response used if the username already exists. I want to show it with console.log("Username Already In Use"), but it doesn't show. What I am missing here to show console.log() anyone can help me?