How to enable AJAX for the the 'Add to Cart' button of product widgets in Magento 2?

You have to override addtocart.phtml file

app/design/frontend/vendorname/themename/Magento_Catalog/templates/product/view/addtocart.phtml

Now Just Replace "bindSubmit": false to "bindSubmit": true

<script type="text/x-magento-init">
    {
        "#product_addtocart_form": {
            "catalogAddToCart": {
                "bindSubmit": false
            }
        }
    }
</script>

To

<script type="text/x-magento-init">
    {
        "#product_addtocart_form": {
            "catalogAddToCart": {
                "bindSubmit": true
            }
        }
    }
</script>

Now delete cache and page_cache folder or flush cache.

Note: Make sure you have to set No from Store->Configuration->Sales->Checkout->Shopping Cart->After Adding a Product Redirect to Shopping Cart to No


You can extend the Catalog Products List to use the ajax cart by creating a module with the following files:

/Your/Module/Block/Product/ListBlock.php with content:

<?php

namespace Your\Module\Block\Product;


use Magento\CatalogWidget\Block\Product\ProductsList;

class ListBlock extends ProductsList
{
    /**
     * @var \Magento\Framework\Url\Helper\Data
     */
    protected $urlHelper;

    public function __construct(
        \Magento\Catalog\Block\Product\Context $context,
        \Magento\Catalog\Model\ResourceModel\Product\CollectionFactory $productCollectionFactory,
        \Magento\Catalog\Model\Product\Visibility $catalogProductVisibility,
        \Magento\Framework\App\Http\Context $httpContext,
        \Magento\Rule\Model\Condition\Sql\Builder $sqlBuilder,
        \Magento\CatalogWidget\Model\Rule $rule,
        \Magento\Widget\Helper\Conditions $conditionsHelper,
        \Magento\Framework\Url\Helper\Data $urlHelper,
        array $data
    )
    {
        $this->urlHelper = $urlHelper;

        parent::__construct($context, $productCollectionFactory, $catalogProductVisibility, $httpContext, $sqlBuilder, $rule, $conditionsHelper, $data);
    }

    /**
     * Get post parameters
     *
     * @param \Magento\Catalog\Model\Product $product
     * @return string
     */
    public function getAddToCartPostParams(\Magento\Catalog\Model\Product $product)
    {
        $url = $this->getAddToCartUrl($product);
        return [
            'action' => $url,
            'data' => [
                'product' => $product->getEntityId(),
                \Magento\Framework\App\ActionInterface::PARAM_NAME_URL_ENCODED =>
                    $this->urlHelper->getEncodedUrl($url),
            ]
        ];
    }
}

Your/Module/etc/widget.xml with content:

<widgets xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Widget:etc/widget.xsd">
    <widget id="ajax_products_list" class="Your\Module\Block\Product\ListBlock" is_email_compatible="true"
            placeholder_image="Magento_CatalogWidget::images/products_list.png" ttl="86400">
        <label translate="true">Ajax Catalog Products List</label>
        <description translate="true">List of Products with Ajax Cart</description>
        <parameters>
            <parameter name="title" xsi:type="text" required="false" visible="true">
                <label translate="true">Title</label>
            </parameter>
            <parameter name="show_pager" xsi:type="select" visible="true"
                       source_model="Magento\Config\Model\Config\Source\Yesno">
                <label translate="true">Display Page Control</label>
            </parameter>
            <parameter name="products_per_page" xsi:type="text" required="true" visible="true">
                <label translate="true">Number of Products per Page</label>
                <depends>
                    <parameter name="show_pager" value="1" />
                </depends>
                <value>5</value>
            </parameter>
            <parameter name="products_count" xsi:type="text" required="true" visible="true">
                <label translate="true">Number of Products to Display</label>
                <value>10</value>
            </parameter>
            <parameter name="template" xsi:type="select" required="true" visible="true">
                <label translate="true">Template</label>
                <options>
                    <option name="default" value="Your_Module::product/widget/content/grid.phtml" selected="true">
                        <label translate="true">Products Grid Template</label>
                    </option>
                </options>
            </parameter>
            <parameter name="cache_lifetime" xsi:type="text" visible="true">
                <label translate="true">Cache Lifetime (Seconds)</label>
                <description translate="true">86400 by default, if not set. To refresh instantly, clear the Blocks HTML Output cache.</description>
            </parameter>
            <parameter name="condition" xsi:type="conditions" visible="true" required="true" sort_order="10"
                       class="Magento\CatalogWidget\Block\Product\Widget\Conditions">
                <label translate="true">Conditions</label>
            </parameter>
        </parameters>
        <containers>
            <container name="content">
                <template name="grid" value="default" />
            </container>
            <container name="content.top">
                <template name="grid" value="default" />
            </container>
            <container name="content.bottom">
                <template name="grid" value="default" />
            </container>
        </containers>
    </widget>
</widgets>

And finally Your/Module/view/frontend/templates/product/widget/content/grid.phtml with content:

<?php

// @codingStandardsIgnoreFile
use Magento\Framework\App\Action\Action;
?>
<?php
/**
 * Template for displaying products list widget
 *
 * @var $block \Your\Module\Block\Product\ListBlock
 */
?>
<?php if ($exist = ($block->getProductCollection() && $block->getProductCollection()->getSize())):?>
<?php
    $type = 'widget-product-grid';

    $mode = 'grid';

    $image = 'new_products_content_widget_grid';
    $title = $block->getTitle() ? __($block->getTitle()) : '';
    $products = $block->getProductCollection()->getItems();
    $_helper = $this->helper('Magento\Catalog\Helper\Output');

    $showWishlist = true;
    $showCompare = true;
    $showCart = true;
    $templateType = \Magento\Catalog\Block\Product\ReviewRendererInterface::DEFAULT_VIEW;
    $showDescription = false;
?>
    <div class="block widget block-products-list <?php /* @escapeNotVerified */ echo $mode; ?>">
        <?php if ($title):?>
        <div class="block-title">
            <strong><?php /* @escapeNotVerified */ echo $title; ?></strong>
        </div>
        <?php endif ?>
        <div class="block-content">
            <?php /* @escapeNotVerified */ echo '<!-- ' . $image . '-->' ?>
            <div class="products-<?php /* @escapeNotVerified */ echo $mode; ?> <?php /* @escapeNotVerified */ echo $mode; ?>">
                <ol class="product-items <?php /* @escapeNotVerified */ echo $type; ?>">
                    <?php $iterator = 1; ?>
                    <?php /** @var \Magento\Catalog\Model\Product $_product */ ?>
                    <?php foreach ($products as $_product): ?>
                        <?php /* @escapeNotVerified */ echo($iterator++ == 1) ? '<li class="product-item">' : '</li><li class="product-item">' ?>
                        <div class="product-item-info" data-container="product-grid">
                            <?php
                            $productImage = $block->getImage($_product, $image);
                            ?>
                            <?php // Product Image ?>
                            <a href="<?php /* @escapeNotVerified */ echo $_product->getProductUrl() ?>" class="product photo product-item-photo" tabindex="-1">
                                <?php echo $productImage->toHtml(); ?>
                            </a>
                            <div class="product details product-item-details">
                                <?php
                                $_productNameStripped = $block->stripTags($_product->getName(), null, true);
                                ?>
                                <strong class="product name product-item-name">
                                    <a class="product-item-link"
                                       href="<?php /* @escapeNotVerified */ echo $_product->getProductUrl() ?>">
                                        <?php /* @escapeNotVerified */ echo $_helper->productAttribute($_product, $_product->getName(), 'name'); ?>
                                    </a>
                                </strong>
                                <?php echo $block->getReviewsSummaryHtml($_product, $templateType); ?>
                                <?php /* @escapeNotVerified */ echo $block->getProductPrice($_product) ?>
                                <?php echo $block->getProductDetailsHtml($_product); ?>

                                <div class="product-item-inner">
                                    <div class="product actions product-item-actions">
                                        <div class="actions-primary">
                                            <?php if ($_product->isSaleable()): ?>
                                                <?php $postParams = $block->getAddToCartPostParams($_product); ?>
                                                <form data-role="tocart-form" action="<?php /* @escapeNotVerified */ echo $postParams['action']; ?>" method="post">
                                                    <input type="hidden" name="product" value="<?php /* @escapeNotVerified */ echo $postParams['data']['product']; ?>">
                                                    <input type="hidden" name="<?php /* @escapeNotVerified */ echo Action::PARAM_NAME_URL_ENCODED; ?>" value="<?php /* @escapeNotVerified */ echo $postParams['data'][Action::PARAM_NAME_URL_ENCODED]; ?>">
                                                    <?php echo $block->getBlockHtml('formkey')?>
                                                    <button type="submit"
                                                            title="<?php echo $block->escapeHtml(__('Add to Cart')); ?>"
                                                            class="action tocart primary">
                                                        <span><?php /* @escapeNotVerified */ echo __('Add to Cart') ?></span>
                                                    </button>
                                                </form>
                                            <?php else: ?>
                                                <?php if ($_product->getIsSalable()): ?>
                                                    <div class="stock available"><span><?php /* @escapeNotVerified */ echo __('In stock') ?></span></div>
                                                <?php else: ?>
                                                    <div class="stock unavailable"><span><?php /* @escapeNotVerified */ echo __('Out of stock') ?></span></div>
                                                <?php endif; ?>
                                            <?php endif; ?>
                                        </div>
                                        <div data-role="add-to-links" class="actions-secondary">
                                            <?php if ($this->helper('Magento\Wishlist\Helper\Data')->isAllow()): ?>
                                                <a href="#"
                                                   class="action towishlist"
                                                   title="<?php echo $block->escapeHtml(__('Add to Wish List')); ?>"
                                                   aria-label="<?php echo $block->escapeHtml(__('Add to Wish List')); ?>"
                                                   data-post='<?php /* @escapeNotVerified */ echo $block->getAddToWishlistParams($_product); ?>'
                                                   data-action="add-to-wishlist"
                                                   role="button">
                                                    <span><?php /* @escapeNotVerified */ echo __('Add to Wish List') ?></span>
                                                </a>
                                            <?php endif; ?>
                                            <?php
                                            $compareHelper = $this->helper('Magento\Catalog\Helper\Product\Compare');
                                            ?>
                                            <a href="#"
                                               class="action tocompare"
                                               title="<?php echo $block->escapeHtml(__('Add to Compare')); ?>"
                                               aria-label="<?php echo $block->escapeHtml(__('Add to Compare')); ?>"
                                               data-post='<?php /* @escapeNotVerified */ echo $compareHelper->getPostDataParams($_product); ?>'
                                               role="button">
                                                <span><?php /* @escapeNotVerified */ echo __('Add to Compare') ?></span>
                                            </a>
                                        </div>
                                    </div>
                                    <?php if ($showDescription):?>
                                        <div class="product description product-item-description">
                                            <?php /* @escapeNotVerified */ echo $_helper->productAttribute($_product, $_product->getShortDescription(), 'short_description') ?>
                                            <a href="<?php /* @escapeNotVerified */ echo $_product->getProductUrl() ?>" title="<?php /* @escapeNotVerified */ echo $_productNameStripped ?>"
                                               class="action more"><?php /* @escapeNotVerified */ echo __('Learn More') ?></a>
                                        </div>
                                    <?php endif; ?>
                                </div>
                            </div>
                        </div>
                        <?php echo($iterator == count($products)+1) ? '</li>' : '' ?>
                    <?php endforeach ?>
                </ol>
            </div>
            <?php echo $block->getPagerHtml() ?>
        </div>
    </div>
    <?php if (!$block->isRedirectToCartEnabled()) : ?>
        <script type="text/x-magento-init">
        {
            "[data-role=tocart-form], .form.map.checkout": {
                "catalogAddToCart": {}
            }
        }
        </script>
    <?php endif; ?>
<?php endif;?>

Be sure and replace all instances of 'Your\Module' with your own namespaces.


Solution for Magento 2.3

For widget Catalog Product list:

You should find template of widget in this case it is

module-catalog-widget/view/frontend/templates/product/widget/content/grid.phtml

and insert this code:

<script type="text/x-magento-init">
    {
        "[data-role=tocart-form], .form.map.checkout": {
            "catalogAddToCart": {}
        }
    }
</script>