tested shared Vuex store, created d8 userblock, enabled JSONAPI module

web/modules/custom/user_block/src/Plugin/Block/UserBlock.php

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+<?php
+
+namespace Drupal\user_block\Plugin\Block;
+
+use Drupal\Core\Session\AccountProxy;
+use Drupal\Core\Session\AccountProxyInterface;
+use Drupal\Core\Block\BlockBase;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Drupal\Core\Url;
+
+/**
+ * Provides a 'UserBlock' block.
+ *
+ * @Block(
+ *  id = "user_block",
+ *  admin_label = @Translation("User block"),
+ * )
+ */
+class UserBlock extends BlockBase   implements ContainerFactoryPluginInterface{
+
+    protected $user;
+
+    /**
+     * {@inheritdoc}
+     */
+    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+      // Instantiates this form class.
+      return new static(
+        $configuration,
+        $plugin_id,
+        $plugin_definition,
+        $container->get('current_user')
+      );
+    }
+
+    /**
+    * @param array $configuration
+    * @param string $plugin_id
+    * @param mixed $plugin_definition
+    * @param \Drupal\Core\Session\AccountProxyInterface $account
+    */
+    public function __construct(array $configuration, $plugin_id, $plugin_definition, AccountProxyInterface $account) {
+      parent::__construct($configuration, $plugin_id, $plugin_definition);
+      $this->user = $account;
+    }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build() {
+    $build = [];
+    // dpm($this->user);
+    if($this->user->id()){
+      $user_url = Url::fromRoute('entity.user.canonical', ['user' => $this->user->id()]);
+      $build['user-link'] = array(
+        '#title' => $this->user->getEmail(),
+        '#type' => 'link',
+        '#url' => $user_url,
+        '#options'=>array(
+          'attributes' => array(
+            'data-drupal-link-system-path' => $user_url->getInternalPath(),
+            'alt' => t('User account'),
+          )
+        )
+      );
+      $logout_url = Url::fromRoute('user.logout');
+      $build['user-logout'] = array(
+        '#title' => t('Logout'),
+        '#type' => 'link',
+        '#url' => $logout_url,
+        '#options'=>array(
+          'attributes' => array(
+            'data-drupal-link-system-path' => $logout_url->getInternalPath(),
+            'alt' => t('Logout'),
+          )
+        )
+      );
+    }
+    return $build;
+  }
+
+}

web/modules/custom/user_block/user_block.info.yml

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+name: 'user_block'
+type: module
+description: 'Provide a block with user name/email and logout link'
+core: 8.x
+package: 'Custom'

web/modules/custom/user_block/user_block.module

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+<?php
+
+/**
+ * @file
+ * Contains user_block.module.
+ */
+
+use Drupal\Core\Routing\RouteMatchInterface;
+
+/**
+ * Implements hook_help().
+ */
+// function user_block_help($route_name, RouteMatchInterface $route_match) {
+//   switch ($route_name) {
+//     // Main module help for the user_block module.
+//     case 'help.page.user_block':
+//       $output = '';
+//       $output .= '<h3>' . t('About') . '</h3>';
+//       $output .= '<p>' . t('Provide a block with user name/email and logout link') . '</p>';
+//       return $output;
+//
+//     default:
+//   }
+// }