Add to Cart – REST API

function add_to_cart_products($request) {
	global $woocommerce, $wpdb;
    $cart_data = get_user_meta( $request['user_id'], '_woocommerce_persistent_cart_' . get_current_blog_id(), true );

	$flag = 0;
	foreach ($cart_data['cart'] as $key => $val) {
        //$_product = $val['data'];
		if ($val['product_id'] != $request['product_id']) {
			$flag = 0;
		} elseif ($val['product_id'] == $request['product_id']) {
			$flag = 2;
			$cart_data['cart'][$key]['quantity']++;
		}
	}
	$string = $woocommerce->cart->generate_cart_id($request['product_id'], 0, array(), $cart_data['cart']);
	$product = wc_get_product($request['product_id']);
	$cart_data['cart'][$string] = array(
		'key' => $string,
		'product_id' => $request['product_id'],
		'variation_id' => 0,
		'variation' => array(),
		'quantity' => $request['quantity'],
		'line_tax_data' => array(
			'subtotal' => array(),
			'total' => array()
		),
		'line_subtotal' => $product->get_price(),
		'line_subtotal_tax' => 0,
		'line_total' => $product->get_price(),
		'line_tax' => 0,
	);
	update_user_meta($request['user_id'], '_woocommerce_persistent_cart_1', $cart_data);
	$saved_cart_meta = get_user_meta($request['user_id'], '_woocommerce_persistent_cart_1', true);
	return new WP_REST_Response($saved_cart_meta);
}

Creating endpoints

add_action('rest_api_init', function () {
	register_rest_route('wp/v2', 'add_to_cart_products', array(
		'methods' => 'POST',
		'callback' => 'add_to_cart_products',
	));
});