问如何将Ajax响应转换为字符串以与JS字符串进行比较

EN

function checkUsn(){
    var usn = document.getElementById("usn").value;
    if(usn){
        $.ajax({
            type: 'post',
            url: 'checkdata.php',
            data: {
                emp_username: usn,
            },
            success: function(response){
                console.log(response);
                myGlobalContainer.usn = response; //convert it to compare with string
                $('#status').html(response);
            }
        });
    }
}

if(myGlobalContainer.usn == "OK"){
return true;
}else{
return false;
}

<?php
header("Content-Type: text/plain");
include 'db_config.php';
$conn = new mysqli($db_servername, $db_username, $db_password, $db_name);

if(isset($_POST['emp_username'])){
    $usn = $_POST['emp_username'];

    $checkdata = "SELECT emp_username FROM emp_details where emp_username='$usn'";

    $query = mysqli_query($conn, $checkdata);

    if(mysqli_num_rows($query) > 0){
        echo "OK";
    }else{
        echo "Your Username not exist";
    }
    exit();
}

if(isset($_POST['emp_pw']) && isset($_POST['emp_usn'])){
    $pw = $_POST['emp_pw'];
    $usn = $_POST['emp_usn'];

    $get_pw = "SELECT emp_password FROM emp_details where emp_username='$usn'";

    $query = mysqli_query($conn, $get_pw);

    //$get_num_rows = mysqli_num_rows($query);
    //echo $get_num_rows;

    $row = mysqli_fetch_assoc($query);
    //echo $row["emp_password"];

    // check if password is match with username
    if($pw == $row["emp_password"]){
        echo "MATCH";
    }else{
        echo "Wrong password";
    }
    exit();
}
?>