diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 187aceb..25f1c6a 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -66,7 +66,11 @@ jobs:
       - name: Run PHPCS
         working-directory: ${{ env.DRUPAL_ROOT }}
         run: |
-          vendor/bin/phpcs $MODULE_FOLDER --standard=Drupal --extensions=php,module,inc,install,test,info
+          vendor/bin/phpcs $MODULE_FOLDER --standard=Drupal,DrupalPractice --extensions=php,module,inc,install,test,info
+
+      - name: Run phpstan
+        working-directory: ${{ env.DRUPAL_ROOT }}
+        run: vendor/bin/phpstan analyze -c $MODULE_FOLDER/phpstan.neon $MODULE_FOLDER
 
       - name: Start services
         working-directory: ${{ env.DRUPAL_ROOT }}
diff --git a/phpstan.neon b/phpstan.neon
new file mode 100644
index 0000000..0c4c2ca
--- /dev/null
+++ b/phpstan.neon
@@ -0,0 +1,12 @@
+parameters:
+  fileExtensions:
+      - php
+      - module
+      - install
+  paths:
+    - ./
+  excludePaths:
+    - vendor
+  level: 3
+  checkMissingIterableValueType: false
+  treatPhpDocTypesAsCertain: false
diff --git a/src/AzureBlobStorageAdapter.php b/src/AzureBlobStorageAdapter.php
index 0b655dd..722c96a 100644
--- a/src/AzureBlobStorageAdapter.php
+++ b/src/AzureBlobStorageAdapter.php
@@ -1,15 +1,6 @@
 <?php
 
-/**
- * @file
- * AzureBlobStorageAdapter.php from league/flysystem-azure-blob-storage:1.0.0.
- * Modified to make this play nicely with Drupal 10 dependencies.
- *
- * @license MIT
- * @license https://github.com/thephpleague/flysystem-azure-blob-storage/blob/1.0.0/LICENSE
- */
-
-// phpcs:ignoreFile
+declare(strict_types = 1);
 
 namespace Drupal\helfi_azure_fs;
 
@@ -31,14 +22,22 @@
 use function stream_get_contents;
 use function strpos;
 
-class AzureBlobStorageAdapter extends AbstractAdapter
-{
+/**
+ * The blob storage adapter.
+ *
+ *  AzureBlobStorageAdapter.php from league/flysystem-azure-blob-storage:1.0.0.
+ *  Modified to make this play nicely with Drupal 10 dependencies.
+ */
+class AzureBlobStorageAdapter extends AbstractAdapter {
+
   use NotSupportingVisibilityTrait;
 
   /**
+   * The meta options.
+   *
    * @var string[]
    */
-  protected static $metaOptions = [
+  protected static array $metaOptions = [
     'CacheControl',
     'ContentType',
     'Metadata',
@@ -47,33 +46,58 @@ class AzureBlobStorageAdapter extends AbstractAdapter
   ];
 
   /**
-   * @var BlobRestProxy
+   * The maximum number of results in content listing.
+   *
+   * @var int
    */
-  private $client;
-
-  private $container;
-
   private $maxResultsForContentsListing = 5000;
 
-  public function __construct(BlobRestProxy $client, $container, $prefix = null)
-  {
-    $this->client = $client;
-    $this->container = $container;
+  /**
+   * Constructs a new instance.
+   *
+   * @param \MicrosoftAzure\Storage\Blob\BlobRestProxy $client
+   *   The client.
+   * @param string $container
+   *   The container.
+   * @param string|null $prefix
+   *   The prefix.
+   */
+  public function __construct(
+    protected BlobRestProxy $client,
+    protected string $container,
+    ?string $prefix = NULL
+  ) {
     $this->setPathPrefix($prefix);
   }
 
-  public function write($path, $contents, Config $config)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function write($path, $contents, Config $config): array {
     return $this->upload($path, $contents, $config) + compact('contents');
   }
 
-  public function writeStream($path, $resource, Config $config)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function writeStream($path, $resource, Config $config): array {
     return $this->upload($path, $resource, $config);
   }
 
-  protected function upload($path, $contents, Config $config)
-  {
+  /**
+   * Upload the given file.
+   *
+   * @param string $path
+   *   The path.
+   * @param string|resource $contents
+   *   The contents.
+   * @param \League\Flysystem\Config $config
+   *   The config object.
+   *
+   * @return array
+   *   The metadata.
+   */
+  protected function upload(string $path, mixed $contents, Config $config): array {
     $destination = $this->applyPathPrefix($path);
 
     $options = $this->getOptionsFromConfig($config);
@@ -82,10 +106,8 @@ protected function upload($path, $contents, Config $config)
       $options->setContentType(Util::guessMimeType($path, $contents));
     }
 
-    /**
-     * We manually create the stream to prevent it from closing the resource
-     * in its destructor.
-     */
+    // We manually create the stream to prevent it from closing the resource
+    // in its destructor.
     $stream = GuzzleUtil::streamFor($contents);
     $response = $this->client->createBlockBlob(
       $this->container,
@@ -104,45 +126,58 @@ protected function upload($path, $contents, Config $config)
     ];
   }
 
-  public function update($path, $contents, Config $config)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function update($path, $contents, Config $config): array {
     return $this->upload($path, $contents, $config) + compact('contents');
   }
 
-  public function updateStream($path, $resource, Config $config)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function updateStream($path, $resource, Config $config): array {
     return $this->upload($path, $resource, $config);
   }
 
-  public function rename($path, $newpath)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function rename($path, $newpath): bool {
     return $this->copy($path, $newpath) && $this->delete($path);
   }
 
-  public function copy($path, $newpath)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function copy($path, $newpath): bool {
     $source = $this->applyPathPrefix($path);
     $destination = $this->applyPathPrefix($newpath);
     $this->client->copyBlob($this->container, $destination, $this->container, $source);
 
-    return true;
+    return TRUE;
   }
 
-  public function delete($path)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function delete($path): bool {
     try {
       $this->client->deleteBlob($this->container, $this->applyPathPrefix($path));
-    } catch (ServiceException $exception) {
+    }
+    catch (ServiceException $exception) {
       if ($exception->getCode() !== 404) {
         throw $exception;
       }
     }
 
-    return true;
+    return TRUE;
   }
 
-  public function deleteDir($dirname)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function deleteDir($dirname): bool {
     $prefix = $this->applyPathPrefix($dirname);
     $options = new ListBlobsOptions();
     $options->setPrefix($prefix . '/');
@@ -151,24 +186,30 @@ public function deleteDir($dirname)
       $this->client->deleteBlob($this->container, $blob->getName());
     }
 
-    return true;
+    return TRUE;
   }
 
-  public function createDir($dirname, Config $config)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function createDir($dirname, Config $config): array {
     return ['path' => $dirname, 'type' => 'dir'];
   }
 
-  public function has($path)
-  {
-    return $this->getMetadata($path);
+  /**
+   * {@inheritdoc}
+   */
+  public function has($path): bool {
+    return (bool) $this->getMetadata($path);
   }
 
-  public function read($path)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function read($path): array {
     $response = $this->readStream($path);
 
-    if ( ! isset($response['stream']) || ! is_resource($response['stream'])) {
+    if (!isset($response['stream']) || !is_resource($response['stream'])) {
       return $response;
     }
 
@@ -178,8 +219,10 @@ public function read($path)
     return $response;
   }
 
-  public function readStream($path)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function readStream($path): array|bool {
     $location = $this->applyPathPrefix($path);
 
     try {
@@ -192,17 +235,20 @@ public function readStream($path)
           $path,
           $response->getProperties()
         ) + ['stream' => $response->getContentStream()];
-    } catch (ServiceException $exception) {
+    }
+    catch (ServiceException $exception) {
       if ($exception->getCode() !== 404) {
         throw $exception;
       }
 
-      return false;
+      return FALSE;
     }
   }
 
-  public function listContents($directory = '', $recursive = false)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function listContents($directory = '', $recursive = FALSE): array {
     $result = [];
     $location = $this->applyPathPrefix($directory);
 
@@ -214,7 +260,7 @@ public function listContents($directory = '', $recursive = false)
     $options->setPrefix($location);
     $options->setMaxResults($this->maxResultsForContentsListing);
 
-    if ( ! $recursive) {
+    if (!$recursive) {
       $options->setDelimiter('/');
     }
 
@@ -229,7 +275,7 @@ public function listContents($directory = '', $recursive = false)
       }
     }
 
-    if ( ! $recursive) {
+    if (!$recursive) {
       $result = array_merge($result, array_map([$this, 'normalizeBlobPrefix'], $response->getBlobPrefixes()));
     }
 
@@ -241,44 +287,70 @@ public function listContents($directory = '', $recursive = false)
     return Util::emulateDirectories($result);
   }
 
-  public function getMetadata($path)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function getMetadata($path): array|bool {
     $path = $this->applyPathPrefix($path);
 
     try {
-      return $this->normalizeBlobProperties(
+      $metadata = $this->normalizeBlobProperties(
         $path,
         $this->client->getBlobProperties($this->container, $path)->getProperties()
       );
-    } catch (ServiceException $exception) {
+
+      if ($metadata === FALSE && in_array($path, ['css', 'js'])) {
+        return [
+          'type' => 'dir',
+          'path' => $path,
+        ];
+      }
+
+      return $metadata;
+    }
+    catch (ServiceException $exception) {
       if ($exception->getCode() !== 404) {
         throw $exception;
       }
 
-      return false;
+      return FALSE;
     }
   }
 
-  public function getSize($path)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function getSize($path): array|bool {
     return $this->getMetadata($path);
   }
 
-  public function getMimetype($path)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function getMimetype($path): array|bool {
     return $this->getMetadata($path);
   }
 
-  public function getTimestamp($path)
-  {
+  /**
+   * {@inheritdoc}
+   */
+  public function getTimestamp($path): array|bool {
     return $this->getMetadata($path);
   }
 
-  protected function getOptionsFromConfig(Config $config)
-  {
+  /**
+   * Gets the blob options.
+   *
+   * @param \League\Flysystem\Config $config
+   *   The config.
+   *
+   * @return mixed
+   *   The options.
+   */
+  protected function getOptionsFromConfig(Config $config) {
     $options = $config->get('blobOptions', new CreateBlockBlobOptions());
     foreach (static::$metaOptions as $option) {
-      if ( ! $config->has($option)) {
+      if (!$config->has($option)) {
         continue;
       }
       call_user_func([$options, "set$option"], $config->get($option));
@@ -290,11 +362,21 @@ protected function getOptionsFromConfig(Config $config)
     return $options;
   }
 
-  protected function normalizeBlobProperties($path, BlobProperties $properties)
-  {
+  /**
+   * Normalizes the given blob properties.
+   *
+   * @param string $path
+   *   The path.
+   * @param \MicrosoftAzure\Storage\Blob\Models\BlobProperties $properties
+   *   The properties.
+   *
+   * @return array
+   *   The normalized properties.
+   */
+  protected function normalizeBlobProperties(string $path, BlobProperties $properties): array {
     $path = $this->removePathPrefix($path);
 
-    if (substr($path, -1) === '/') {
+    if (str_ends_with($path, '/')) {
       return ['type' => 'dir', 'path' => rtrim($path, '/')];
     }
 
@@ -309,15 +391,29 @@ protected function normalizeBlobProperties($path, BlobProperties $properties)
   }
 
   /**
+   * Sets the maximum number of results for content listing.
+   *
    * @param int $numberOfResults
+   *   The number of results.
    */
-  public function setMaxResultsForContentsListing($numberOfResults)
-  {
+  public function setMaxResultsForContentsListing(int $numberOfResults): void {
     $this->maxResultsForContentsListing = $numberOfResults;
   }
 
-  protected function normalizeBlobPrefix(BlobPrefix $blobPrefix)
-  {
-    return ['type' => 'dir', 'path' => $this->removePathPrefix(rtrim($blobPrefix->getName(), '/'))];
+  /**
+   * Normalizes the given blob prefix.
+   *
+   * @param \MicrosoftAzure\Storage\Blob\Models\BlobPrefix $blobPrefix
+   *   The prefix.
+   *
+   * @return array
+   *   The normalized blob prefix.
+   */
+  protected function normalizeBlobPrefix(BlobPrefix $blobPrefix): array {
+    return [
+      'type' => 'dir',
+      'path' => $this->removePathPrefix(rtrim($blobPrefix->getName(), '/')),
+    ];
   }
+
 }
diff --git a/src/Flysystem/Adapter/AzureBlobStorageAdapter.php b/src/Flysystem/Adapter/AzureBlobStorageAdapter.php
index bacde2a..f307fc3 100644
--- a/src/Flysystem/Adapter/AzureBlobStorageAdapter.php
+++ b/src/Flysystem/Adapter/AzureBlobStorageAdapter.php
@@ -14,7 +14,7 @@ class AzureBlobStorageAdapter extends AzureBlobStorageAdapterBase {
   /**
    * {@inheritdoc}
    */
-  public function getMetadata($path) {
+  public function getMetadata($path): bool|array {
     $metadata = parent::getMetadata($path);
 
     if ($metadata === FALSE && in_array($path, ['css', 'js'])) {