Customer.php

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace Magento\Customer\CustomerData;

use Magento\Customer\Helper\Session\CurrentCustomer;
use Magento\Customer\Helper\View;

/**
 * Customer section
 */
class Customer implements SectionSourceInterface
{
    /**
     * @var CurrentCustomer
     */
    protected $currentCustomer;

    /**
     * @var View
     */
    private $customerViewHelper;

    /**
     * @param CurrentCustomer $currentCustomer
     * @param View $customerViewHelper
     */
    public function __construct(
        CurrentCustomer $currentCustomer,
        View $customerViewHelper
    ) {
        $this->currentCustomer = $currentCustomer;
        $this->customerViewHelper = $customerViewHelper;
    }

    /**
     * {@inheritdoc}
     */
    public function getSectionData()
    {
        if (!$this->currentCustomer->getCustomerId()) {
            return [];
        }

        $customer = $this->currentCustomer->getCustomer();
        return [
            'fullname' => $this->customerViewHelper->getCustomerName($customer),
            'firstname' => $customer->getFirstname(),
            'websiteId' => $customer->getWebsiteId(),
        ];
    }
}