As per my requirements, I have created this type of controller. So basically you need 2 types of Controller.

The first one for adding the product in the comparison list and second for removing the product from the comparison list.

Step 1. Please create one controller for Add Product as below.

<?php

namespace Magemonkeys\Compareajax\Controller\Index;

use Magento\Catalog\Api\ProductRepositoryInterface;
use Magento\Framework\Data\Form\FormKey\Validator;
use Magento\Framework\View\Result\PageFactory;
use \Magento\Framework\Controller\Result\JsonFactory;
use Magento\Framework\Exception\NoSuchEntityException;
use Magento\Framework\Event\ManagerInterface as EventManager;

class AddCompare extends \Magento\Framework\App\Action\Action
{
    /**
     * @var JsonFactory
     */
    protected $resultJsonFactory;

    /**
    * @var EventManager
    */
    private $eventManager;

    /**
     * Customer id
     *
     * @var null|int
     */
    protected $_customerId = null;

    /**
     * Catalog session
     *
     * @var \Magento\Catalog\Model\Session
     */
    protected $_catalogSession;

    /**
     * Catalog product compare list
     *
     * @var \Magento\Catalog\Model\Product\Compare\ListCompare
     */
    protected $_catalogProductCompareList;

    /**
     * Customer visitor
     *
     * @var \Magento\Customer\Model\Visitor
     */
    protected $_customerVisitor;

    /**
     * Customer session
     *
     * @var \Magento\Customer\Model\Session
     */
    protected $_customerSession;

    /**
     * Item collection factory
     *
     * @var \Magento\Catalog\Model\ResourceModel\Product\Compare\Item\CollectionFactory
     */
    protected $_itemCollectionFactory;

    /**
     * Compare item factory
     *
     * @var \Magento\Catalog\Model\Product\Compare\ItemFactory
     */
    protected $_compareItemFactory;

    /**
     * @var \Magento\Store\Model\StoreManagerInterface
     */
    protected $_storeManager;

    /**
     * @var Validator
     */
    protected $_formKeyValidator;

    /**
     * @var \Magento\Framework\View\Result\PageFactory
     */
    protected $resultPageFactory;

    /**
     * @var ProductRepositoryInterface
     */
    protected $productRepository;

    protected $_compareHelper;

    protected $_urlInterface;

    protected $_blockFactory;

    public function __construct(
        \Magento\Framework\App\Action\Context $context,
        \Magento\Catalog\Model\Product\Compare\ItemFactory $compareItemFactory,
        \Magento\Catalog\Model\ResourceModel\Product\Compare\Item\CollectionFactory $itemCollectionFactory,
        \Magento\Customer\Model\Session $customerSession,
        \Magento\Customer\Model\Visitor $customerVisitor,
        \Magento\Catalog\Model\Product\Compare\ListCompare $catalogProductCompareList,
        \Magento\Catalog\Model\Session $catalogSession,
        \Magento\Store\Model\StoreManagerInterface $storeManager,
        Validator $formKeyValidator,
        PageFactory $resultPageFactory,
        ProductRepositoryInterface $productRepository,
        JsonFactory $resultJsonFactory,
        EventManager $eventManager,
        \Magento\Catalog\Helper\Product\Compare $compareHelper,
        \Magento\Framework\UrlInterface $urlInterface,
        \Magento\Framework\View\Element\BlockFactory $blockFactory
    ) {
        $this->resultJsonFactory = $resultJsonFactory;
        $this->_storeManager = $storeManager;
        $this->_compareItemFactory = $compareItemFactory;
        $this->_itemCollectionFactory = $itemCollectionFactory;
        $this->_customerSession = $customerSession;
        $this->_customerVisitor = $customerVisitor;
        $this->_catalogProductCompareList = $catalogProductCompareList;
        $this->_catalogSession = $catalogSession;
        $this->_formKeyValidator = $formKeyValidator;
        $this->resultPageFactory = $resultPageFactory;
        $this->productRepository = $productRepository;
        $this->eventManager = $eventManager;
        $this->_compareHelper = $compareHelper;
        $this->_urlInterface = $urlInterface;
        $this->_blockFactory = $blockFactory;
        parent::__construct($context);
    }

    public function execute()
    {
        $resultJson = $this->resultJsonFactory->create();

        $result = array();
        $error = true;
        $message = '';

        // TODO: Validate this is a POST request.

        $count = $this->_compareHelper->getItemCount();
        if($count >= 4) {
            $message = __('You can add the compared products under 4 item(s)');
            $error = true;
            return $resultJson->setData([
                'message' => $message,
                'data' => $result,
                'error' => $error
            ]);
        }

        $productId = (int)$this->getRequest()->getParam('product');
        if ($productId && ($this->_customerVisitor->getId() || $this->_customerSession->isLoggedIn())) {
            $storeId = $this->_storeManager->getStore()->getId();
            try {
                $product = $this->productRepository->getById($productId, false, $storeId);
            } catch (NoSuchEntityException $e) {
                $product = null;
                $message = __('Product does not exist');
                $error = true;
            }

            if ($product) {
                $this->_catalogProductCompareList->addProduct($product);
                $this->_eventManager->dispatch('catalog_product_compare_add_product', ['product' => $product]);
            }

            $this->_objectManager->get('Magento\Catalog\Helper\Product\Compare')->calculate();
            $html = '';
            if($this->_compareHelper->hasItems()){ 
                $compItem = $this->_compareHelper->getItemCollection();
                $i = 1;
                $responseArray = array();
                foreach($compItem->getData() as $comitem){
                    $productNew = $this->_objectManager->get('Magento\Catalog\Model\Product')->load($comitem['entity_id']);
                    $imageBlock = $this->_blockFactory->createBlock('Magento\Catalog\Block\Product\ListProduct');
                    $productImage = $imageBlock->getImage($productNew, 'product_comparison_list');
                    $imageUrl = $productImage->getImageUrl();
                    $responseArray[$i]['id'] = $comitem["entity_id"];
                    $responseArray[$i]['url'] = $imageUrl;
                    $i++;
                }

                for ($j = 1; $j <= 4; $j++)
                {
                    if(isset($responseArray[$j]))
                    {
                        $html .= '<div class="compare-item active" data-itemid="'.$responseArray[$j]['id'].'">
                            <div class="compare-item-number">'. $j .'</div>
                            <a class="compare-item-remove removecompareajaxlink" href="javascript:void(0)" data-productid="'.$responseArray[$j]['id'].'" data-url="'. $this->_urlInterface->getUrl().'compareajax/index/removeCompare/product/'.$responseArray[$j]['id'] .'?isAjax=true"><i class="fa fa-remove"></i></a>
                            <img src="'. $responseArray[$j]['url'] .'" class="imageCompare" />
                        </div>';
                    }
                    else
                    {
                        $html .= '<div class="compare-item active" data-itemid="">
                            <div class="compare-item-number">'. $j .'</div>
                        </div>';
                    }
                }
            }
            $message = __('Product successfully added in comparison list');
            $result = array('html'=>$html,'id'=>$productId);
            $error = false;
        }

        return $resultJson->setData([
                    'message' => $message,
                    'data' => $result,
                    'error' => $error
        ]);
    }
}

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<?php

namespace Magemonkeys\Compareajax\Controller\Index;

use Magento\Catalog\Api\ProductRepositoryInterface;

use Magento\Framework\Data\Form\FormKey\Validator;

use Magento\Framework\View\Result\PageFactory;

use \Magento\Framework\Controller\Result\JsonFactory;

use Magento\Framework\Exception\NoSuchEntityException;

use Magento\Framework\Event\ManagerInterface as EventManager;

class AddCompare extends \Magento\Framework\App\Action\Action

{

/**

* @var JsonFactory

protected $resultJsonFactory;

/**

* @var EventManager

private $eventManager;

/**

* Customer id

* @var null|int

protected $_customerId = null;

/**

* Catalog session

* @var \Magento\Catalog\Model\Session

protected $_catalogSession;

/**

* Catalog product compare list

* @var \Magento\Catalog\Model\Product\Compare\ListCompare

protected $_catalogProductCompareList;

/**

* Customer visitor

* @var \Magento\Customer\Model\Visitor

protected $_customerVisitor;

/**

* Customer session

* @var \Magento\Customer\Model\Session

protected $_customerSession;

/**

* Item collection factory

* @var \Magento\Catalog\Model\ResourceModel\Product\Compare\Item\CollectionFactory

protected $_itemCollectionFactory;

/**

* Compare item factory

* @var \Magento\Catalog\Model\Product\Compare\ItemFactory

protected $_compareItemFactory;

/**

* @var \Magento\Store\Model\StoreManagerInterface

protected $_storeManager;

/**

* @var Validator

protected $_formKeyValidator;

/**

* @var \Magento\Framework\View\Result\PageFactory

protected $resultPageFactory;

/**

* @var ProductRepositoryInterface

protected $productRepository;

protected $_compareHelper;

protected $_urlInterface;

protected $_blockFactory;

public function __construct(

\Magento\Framework\App\Action\Context $context,

\Magento\Catalog\Model\Product\Compare\ItemFactory $compareItemFactory,

\Magento\Catalog\Model\ResourceModel\Product\Compare\Item\CollectionFactory $itemCollectionFactory,

\Magento\Customer\Model\Session $customerSession,

\Magento\Customer\Model\Visitor $customerVisitor,

\Magento\Catalog\Model\Product\Compare\ListCompare $catalogProductCompareList,

\Magento\Catalog\Model\Session $catalogSession,

\Magento\Store\Model\StoreManagerInterface $storeManager,

Validator $formKeyValidator,

PageFactory $resultPageFactory,

ProductRepositoryInterface $productRepository,

JsonFactory $resultJsonFactory,

EventManager $eventManager,

\Magento\Catalog\Helper\Product\Compare $compareHelper,

\Magento\Framework\UrlInterface $urlInterface,

\Magento\Framework\View\Element\BlockFactory $blockFactory

) {

$this->resultJsonFactory = $resultJsonFactory;

$this->_storeManager = $storeManager;

$this->_compareItemFactory = $compareItemFactory;

$this->_itemCollectionFactory = $itemCollectionFactory;

$this->_customerSession = $customerSession;

$this->_customerVisitor = $customerVisitor;

$this->_catalogProductCompareList = $catalogProductCompareList;

$this->_catalogSession = $catalogSession;

$this->_formKeyValidator = $formKeyValidator;

$this->resultPageFactory = $resultPageFactory;

$this->productRepository = $productRepository;

$this->eventManager = $eventManager;

$this->_compareHelper = $compareHelper;

$this->_urlInterface = $urlInterface;

$this->_blockFactory = $blockFactory;

parent::__construct($context);

}

public function execute()

{

$resultJson = $this->resultJsonFactory->create();

$result = array();

$error = true;

$message = '';

// TODO: Validate this is a POST request.

$count = $this->_compareHelper->getItemCount();

if($count >= 4) {

$message = __('You can add the compared products under 4 item(s)');

$error = true;

return $resultJson->setData([

'message' => $message,

'data' => $result,

'error' => $error

]);

}

$productId = (int)$this->getRequest()->getParam('product');

if ($productId && ($this->_customerVisitor->getId() || $this->_customerSession->isLoggedIn())) {

$storeId = $this->_storeManager->getStore()->getId();

try {

$product = $this->productRepository->getById($productId, false, $storeId);

} catch (NoSuchEntityException $e) {

$product = null;

$message = __('Product does not exist');

$error = true;

}

if ($product) {

$this->_catalogProductCompareList->addProduct($product);

$this->_eventManager->dispatch('catalog_product_compare_add_product', ['product' => $product]);

}

$this->_objectManager->get('Magento\Catalog\Helper\Product\Compare')->calculate();

$html = '';

if($this->_compareHelper->hasItems()){

$compItem = $this->_compareHelper->getItemCollection();

$i = 1;

$responseArray = array();

foreach($compItem->getData() as $comitem){

$productNew = $this->_objectManager->get('Magento\Catalog\Model\Product')->load($comitem['entity_id']);

$imageBlock = $this->_blockFactory->createBlock('Magento\Catalog\Block\Product\ListProduct');

$productImage = $imageBlock->getImage($productNew, 'product_comparison_list');

$imageUrl = $productImage->getImageUrl();

$responseArray[$i]['id'] = $comitem["entity_id"];

$responseArray[$i]['url'] = $imageUrl;

$i++;

}

for ($j = 1; $j <= 4; $j++)

{

if(isset($responseArray[$j]))

{

$html .= '<div class="compare-item active" data-itemid="'.$responseArray[$j]['id'].'">

<a class="compare-item-remove removecompareajaxlink" href="javascript:void(0)" data-productid="'.$responseArray[$j]['id'].'" data-url="'. $this->_urlInterface->getUrl().'compareajax/index/removeCompare/product/'.$responseArray[$j]['id'] .'?isAjax=true"><i class="fa fa-remove"></i></a>

</div>';

}

else

{

$html .= '<div class="compare-item active" data-itemid="">

</div>';

}

$message = __('Product successfully added in comparison list');

$result = array('html'=>$html,'id'=>$productId);

$error = false;

}

return $resultJson->setData([

'message' => $message,

'data' => $result,

'error' => $error

]);

}

Step 2. Please create another controller for removing the product as below.

<?php

namespace Magemonkeys\Compareajax\Controller\Index;

use Magento\Catalog\Api\ProductRepositoryInterface;
use Magento\Framework\Data\Form\FormKey\Validator;
use Magento\Framework\View\Result\PageFactory;
use \Magento\Framework\Controller\Result\JsonFactory;
use Magento\Framework\Exception\NoSuchEntityException;
use Magento\Framework\Event\ManagerInterface as EventManager;

class RemoveCompare extends \Magento\Framework\App\Action\Action
{
    /**
     * @var JsonFactory
     */
    protected $resultJsonFactory;

    /**
    * @var EventManager
    */
    private $eventManager;

    /**
     * Customer id
     *
     * @var null|int
     */
    protected $_customerId = null;

    /**
     * Catalog session
     *
     * @var \Magento\Catalog\Model\Session
     */
    protected $_catalogSession;

    /**
     * Catalog product compare list
     *
     * @var \Magento\Catalog\Model\Product\Compare\ListCompare
     */
    protected $_catalogProductCompareList;

    /**
     * Customer visitor
     *
     * @var \Magento\Customer\Model\Visitor
     */
    protected $_customerVisitor;

    /**
     * Customer session
     *
     * @var \Magento\Customer\Model\Session
     */
    protected $_customerSession;

    /**
     * Item collection factory
     *
     * @var \Magento\Catalog\Model\ResourceModel\Product\Compare\Item\CollectionFactory
     */
    protected $_itemCollectionFactory;

    /**
     * Compare item factory
     *
     * @var \Magento\Catalog\Model\Product\Compare\ItemFactory
     */
    protected $_compareItemFactory;

    /**
     * @var \Magento\Store\Model\StoreManagerInterface
     */
    protected $_storeManager;

    /**
     * @var Validator
     */
    protected $_formKeyValidator;

    /**
     * @var \Magento\Framework\View\Result\PageFactory
     */
    protected $resultPageFactory;

    /**
     * @var ProductRepositoryInterface
     */
    protected $productRepository;

    protected $_compareHelper;

    protected $_urlInterface;

    protected $_blockFactory;

    public function __construct(
        \Magento\Framework\App\Action\Context $context,
        \Magento\Catalog\Model\Product\Compare\ItemFactory $compareItemFactory,
        \Magento\Catalog\Model\ResourceModel\Product\Compare\Item\CollectionFactory $itemCollectionFactory,
        \Magento\Customer\Model\Session $customerSession,
        \Magento\Customer\Model\Visitor $customerVisitor,
        \Magento\Catalog\Model\Product\Compare\ListCompare $catalogProductCompareList,
        \Magento\Catalog\Model\Session $catalogSession,
        \Magento\Store\Model\StoreManagerInterface $storeManager,
        Validator $formKeyValidator,
        PageFactory $resultPageFactory,
        ProductRepositoryInterface $productRepository,
        JsonFactory $resultJsonFactory,
        EventManager $eventManager,
        \Magento\Catalog\Helper\Product\Compare $compareHelper,
        \Magento\Framework\UrlInterface $urlInterface,
        \Magento\Framework\View\Element\BlockFactory $blockFactory
    ) {
        $this->resultJsonFactory = $resultJsonFactory;
        $this->_storeManager = $storeManager;
        $this->_compareItemFactory = $compareItemFactory;
        $this->_itemCollectionFactory = $itemCollectionFactory;
        $this->_customerSession = $customerSession;
        $this->_customerVisitor = $customerVisitor;
        $this->_catalogProductCompareList = $catalogProductCompareList;
        $this->_catalogSession = $catalogSession;
        $this->_formKeyValidator = $formKeyValidator;
        $this->resultPageFactory = $resultPageFactory;
        $this->productRepository = $productRepository;
        $this->eventManager = $eventManager;
        $this->_compareHelper = $compareHelper;
        $this->_urlInterface = $urlInterface;
        $this->_blockFactory = $blockFactory;
        parent::__construct($context);
    }

    public function execute()
    {
        $resultJson = $this->resultJsonFactory->create();

        $result = array();
        $error = true;
        $message = '';

        // TODO: Validate this is a POST request.

        $productId = (int)$this->getRequest()->getParam('product');
        if ($productId) {
            $storeId = $this->_storeManager->getStore()->getId();
            try {
                $product = $this->productRepository->getById($productId, false, $storeId);
            } catch (NoSuchEntityException $e) {
                $product = null;
                $message = __('Product does not exist');
                $error = true;
            }

            if ($product) {
                $item = $this->_compareItemFactory->create();
                if ($this->_customerSession->isLoggedIn()) {
                    $item->setCustomerId($this->_customerSession->getCustomerId());
                } elseif ($this->_customerId) {
                    $item->setCustomerId($this->_customerId);
                } else {
                    $item->addVisitorId($this->_customerVisitor->getId());
                }

                $item->loadByProduct($product);

                if ($item->getId()) {
                    $item->delete();
                    $productName = $this->_objectManager->get(\Magento\Framework\Escaper::class)
                        ->escapeHtml($product->getName());
                    $message = __('You removed product %1 from the comparison list.', $productName);
                    $this->_eventManager->dispatch('catalog_product_compare_remove_product',['product' => $item]);
                    $this->_objectManager->get('Magento\Catalog\Helper\Product\Compare')->calculate();
                }

                $html = '';
                if($this->_compareHelper->hasItems()){ 
                    $compItem = $this->_compareHelper->getItemCollection();
                    $i = 1;
                    $responseArray = array();
                    foreach($compItem->getData() as $comitem){
                        $productNew = $this->_objectManager->get('Magento\Catalog\Model\Product')->load($comitem['entity_id']);
                        $imageBlock = $this->_blockFactory->createBlock('Magento\Catalog\Block\Product\ListProduct');
                        $productImage = $imageBlock->getImage($productNew, 'product_comparison_list');
                        $imageUrl = $productImage->getImageUrl();
                        $responseArray[$i]['id'] = $comitem["entity_id"];
                        $responseArray[$i]['url'] = $imageUrl;
                        $i++;
                    }

                    for ($j = 1; $j <= 4; $j++)
                    {
                        if(isset($responseArray[$j]))
                        {
                            $html .= '<div class="compare-item active" data-itemid="'.$responseArray[$j]['id'].'">
                                <div class="compare-item-number">'. $j .'</div>
                                <a class="compare-item-remove removecompareajaxlink" href="javascript:void(0)" data-productid="'.$responseArray[$j]['id'].'" data-url="'. $this->_urlInterface->getUrl().'compareajax/index/removeCompare/product/'.$responseArray[$j]['id'] .'?isAjax=true"><i class="fa fa-remove"></i></a>
                                <img src="'. $responseArray[$j]['url'] .'" class="imageCompare" />
                            </div>';
                        }
                        else
                        {
                            $html .= '<div class="compare-item active" data-itemid="">
                                <div class="compare-item-number">'. $j .'</div>
                            </div>';
                        }
                    }
                }
                $message = __('You removed product from the comparison list.');
                $result = array('html'=>$html,'id'=>$productId);
                $error = false;
            }

        }

        return $resultJson->setData([
                    'message' => $message,
                    'data' => $result,
                    'error' => $error
        ]);
    }
}

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<?php

namespace Magemonkeys\Compareajax\Controller\Index;

use Magento\Catalog\Api\ProductRepositoryInterface;

use Magento\Framework\Data\Form\FormKey\Validator;

use Magento\Framework\View\Result\PageFactory;

use \Magento\Framework\Controller\Result\JsonFactory;

use Magento\Framework\Exception\NoSuchEntityException;

use Magento\Framework\Event\ManagerInterface as EventManager;

class RemoveCompare extends \Magento\Framework\App\Action\Action

{

/**

* @var JsonFactory

protected $resultJsonFactory;

/**

* @var EventManager

private $eventManager;

/**

* Customer id

* @var null|int

protected $_customerId = null;

/**

* Catalog session

* @var \Magento\Catalog\Model\Session

protected $_catalogSession;

/**

* Catalog product compare list

* @var \Magento\Catalog\Model\Product\Compare\ListCompare

protected $_catalogProductCompareList;

/**

* Customer visitor

* @var \Magento\Customer\Model\Visitor

protected $_customerVisitor;

/**

* Customer session

* @var \Magento\Customer\Model\Session

protected $_customerSession;

/**

* Item collection factory

* @var \Magento\Catalog\Model\ResourceModel\Product\Compare\Item\CollectionFactory

protected $_itemCollectionFactory;

/**

* Compare item factory

* @var \Magento\Catalog\Model\Product\Compare\ItemFactory

protected $_compareItemFactory;

/**

* @var \Magento\Store\Model\StoreManagerInterface

protected $_storeManager;

/**

* @var Validator

protected $_formKeyValidator;

/**

* @var \Magento\Framework\View\Result\PageFactory

protected $resultPageFactory;

/**

* @var ProductRepositoryInterface

protected $productRepository;

protected $_compareHelper;

protected $_urlInterface;

protected $_blockFactory;

public function __construct(

\Magento\Framework\App\Action\Context $context,

\Magento\Catalog\Model\Product\Compare\ItemFactory $compareItemFactory,

\Magento\Catalog\Model\ResourceModel\Product\Compare\Item\CollectionFactory $itemCollectionFactory,

\Magento\Customer\Model\Session $customerSession,

\Magento\Customer\Model\Visitor $customerVisitor,

\Magento\Catalog\Model\Product\Compare\ListCompare $catalogProductCompareList,

\Magento\Catalog\Model\Session $catalogSession,

\Magento\Store\Model\StoreManagerInterface $storeManager,

Validator $formKeyValidator,

PageFactory $resultPageFactory,

ProductRepositoryInterface $productRepository,

JsonFactory $resultJsonFactory,

EventManager $eventManager,

\Magento\Catalog\Helper\Product\Compare $compareHelper,

\Magento\Framework\UrlInterface $urlInterface,

\Magento\Framework\View\Element\BlockFactory $blockFactory

) {

$this->resultJsonFactory = $resultJsonFactory;

$this->_storeManager = $storeManager;

$this->_compareItemFactory = $compareItemFactory;

$this->_itemCollectionFactory = $itemCollectionFactory;

$this->_customerSession = $customerSession;

$this->_customerVisitor = $customerVisitor;

$this->_catalogProductCompareList = $catalogProductCompareList;

$this->_catalogSession = $catalogSession;

$this->_formKeyValidator = $formKeyValidator;

$this->resultPageFactory = $resultPageFactory;

$this->productRepository = $productRepository;

$this->eventManager = $eventManager;

$this->_compareHelper = $compareHelper;

$this->_urlInterface = $urlInterface;

$this->_blockFactory = $blockFactory;

parent::__construct($context);

}

public function execute()

{

$resultJson = $this->resultJsonFactory->create();

$result = array();

$error = true;

$message = '';

// TODO: Validate this is a POST request.

$productId = (int)$this->getRequest()->getParam('product');

if ($productId) {

$storeId = $this->_storeManager->getStore()->getId();

try {

$product = $this->productRepository->getById($productId, false, $storeId);

} catch (NoSuchEntityException $e) {

$product = null;

$message = __('Product does not exist');

$error = true;

}

if ($product) {

$item = $this->_compareItemFactory->create();

if ($this->_customerSession->isLoggedIn()) {

$item->setCustomerId($this->_customerSession->getCustomerId());

} elseif ($this->_customerId) {

$item->setCustomerId($this->_customerId);

} else {

$item->addVisitorId($this->_customerVisitor->getId());

}

$item->loadByProduct($product);

if ($item->getId()) {

$item->delete();

$productName = $this->_objectManager->get(\Magento\Framework\Escaper::class)

->escapeHtml($product->getName());

$message = __('You removed product %1 from the comparison list.', $productName);

$this->_eventManager->dispatch('catalog_product_compare_remove_product',['product' => $item]);

$this->_objectManager->get('Magento\Catalog\Helper\Product\Compare')->calculate();

}

$html = '';

if($this->_compareHelper->hasItems()){

$compItem = $this->_compareHelper->getItemCollection();

$i = 1;

$responseArray = array();

foreach($compItem->getData() as $comitem){

$productNew = $this->_objectManager->get('Magento\Catalog\Model\Product')->load($comitem['entity_id']);

$imageBlock = $this->_blockFactory->createBlock('Magento\Catalog\Block\Product\ListProduct');

$productImage = $imageBlock->getImage($productNew, 'product_comparison_list');

$imageUrl = $productImage->getImageUrl();

$responseArray[$i]['id'] = $comitem["entity_id"];

$responseArray[$i]['url'] = $imageUrl;

$i++;

}

for ($j = 1; $j <= 4; $j++)

{

if(isset($responseArray[$j]))

{

$html .= '<div class="compare-item active" data-itemid="'.$responseArray[$j]['id'].'">

</div>';

}

else

{

$html .= '<div class="compare-item active" data-itemid="">

</div>';

}

$message = __('You removed product from the comparison list.');

$result = array('html'=>$html,'id'=>$productId);

$error = false;

}

return $resultJson->setData([

'message' => $message,

'data' => $result,

'error' => $error

]);

}

You can execute and modify these controllers as per your requirements.

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