Im trying to get data from my php file using success function(data)

When im using console.log(data) i get; true/false/exists But i cant make a if statement, if i do it dosent alert!!

Heres my js file

function ajaxCall(username, email, password){

$.ajax({
    type: 'POST',
    url: '../register.php',
    data:   {
        'username' : username,
        'email' : email,
        'password' : password,
    },
    success: function(data) {
     if(data === "exists")
     alert("user exists");
    }
});

}

heres my .php file

<?php

$servername = "#";
$username = "#";
$password = "#";
$dbname = "#";

$regusername = $_POST['username'];
$email = $_POST['email'];
$regpassword = $_POST['password'];   

$mysqli = new mysqli($servername, $username, $password, $dbname);
$selectQuery = "SELECT username FROM users WHERE username = '$regusername'";
$select = $mysqli->query($selectQuery);
// var_dump($select->num_rows);

if( $select->num_rows == 0) {
    $insertQuery = "INSERT INTO users (username, password, email) VALUES ('$regusername','$regpassword','$email');";
    $insert = $mysqli->query($insertQuery);

        if( $insert == true) {
            echo "true";
        }else {
            echo "false";
        }
}else {
    echo "exists";
}

?>

Use a properly structured data format, such as json:

php:

$data=['exists'=>false];
if( $select->num_rows == 0) {
    $insertQuery = "INSERT INTO users (username, password, email) VALUES ('$regusername','$regpassword','$email');";
    $data['inserted'] = (bool) $mysqli->query($insertQuery);
}else {
    $data['exists']=true;
}
header('Content-Type: application/json');
echo json_encode($data);
die();

JS

$.ajax({
    type: 'POST',
    url: '../register.php',
    data:   {
        'username' : username,
        'email' : email,
        'password' : password,
    },
    success: function(data) {
       if(data.exists){
           alert("user exists");
       }elseif(data.inserted){
           alert("inserted");
       }else{
           alert("did not insert");
       }
});

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