add unit test to confirm core functionality

tests/src/Functional/JsonApiLinksFunctionalTest.php

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\jsonapi\Functional;
+
+use Drupal\Component\Serialization\Json;
+
+/**
+ * General functional test class.
+ *
+ * @group jsonapi_links
+ */
+class JsonApiLinksFunctionalTest extends JsonApiFunctionalTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'basic_auth',
+    'jsonapi_links',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Tests the GET method.
+   */
+  public function testRead(): void {
+    // First, do testRead() from core's JSON:API and ensure that the output is
+    // exactly the same as core.
+    // JSON:API Links does not remove anything by default.
+    $this->createDefaultContent(61, 5, TRUE, TRUE, static::IS_NOT_MULTILINGUAL, FALSE);
+    // Unpublish the last entity, so we can check access.
+    $this->nodes[60]->setUnpublished()->save();
+
+    $uuid = $this->nodes[0]->uuid();
+
+    // 0. HEAD request allows a client to verify that JSON:API is installed.
+    $this->httpClient->request('HEAD', $this->buildUrl('/jsonapi/node/article'));
+    $this->assertSession()->statusCodeEquals(200);
+
+    // Confirm the default behavior (no change from core).
+    $this->checkForLinksPresent($uuid, FALSE);
+
+    // Enable link removal with JSON:API Links.
+    \Drupal::configFactory()->getEditable('jsonapi_links.settings')
+      ->set('remove_links', TRUE)
+      ->save();
+
+    // Verify that the configuration value is set.
+    $config = $this->config('jsonapi_links.settings');
+    $this->assertEquals(TRUE, $config->get('remove_links'));
+
+    // Ensure links are removed as expected.
+    $this->checkForLinksPresent($uuid, TRUE);
+
+    // Disable link removal with JSON:API Links.
+    \Drupal::configFactory()->getEditable('jsonapi_links.settings')
+      ->set('remove_links', FALSE)
+      ->save();
+
+    // Verify that the configuration value is set.
+    $config = $this->config('jsonapi_links.settings');
+    $this->assertEquals(FALSE, $config->get('remove_links'));
+
+    // Ensure that the prior behavior has been restored.
+    $this->checkForLinksPresent($uuid, FALSE);
+  }
+
+  /**
+   * Confirms that link attributes are present or absent.
+   *
+   * @param string $uuid
+   *   The uuid of the article.
+   * @param bool $links_removed
+   *   TRUE if links should be removed; FALSE otherwise.
+   */
+  private function checkForLinksPresent(string $uuid, bool $links_removed): void {
+    // 6. Single relationship item.
+    $single_output = Json::decode($this->drupalGet('/jsonapi/node/article/' . $uuid . '/relationships/node_type'));
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertArrayHasKey('type', $single_output['data']);
+    $this->assertArrayNotHasKey('attributes', $single_output['data']);
+    $this->assertLinksRemoved($single_output, $links_removed);
+    // 7. Single relationship image.
+    $single_output = Json::decode($this->drupalGet('/jsonapi/node/article/' . $uuid . '/relationships/field_image'));
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertArrayHasKey('type', $single_output['data']);
+    $this->assertArrayNotHasKey('attributes', $single_output['data']);
+    $this->assertLinksRemoved($single_output, $links_removed);
+    // 8. Multiple relationship item.
+    $single_output = Json::decode($this->drupalGet('/jsonapi/node/article/' . $uuid . '/relationships/field_tags'));
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertArrayHasKey('type', $single_output['data'][0]);
+    $this->assertArrayNotHasKey('attributes', $single_output['data'][0]);
+    $this->assertLinksRemoved($single_output, $links_removed);
+    // 10. Single article with includes.
+    $single_output = Json::decode($this->drupalGet('/jsonapi/node/article/' . $uuid, [
+      'query' => ['include' => 'uid,field_tags'],
+    ]));
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertEquals('node--article', $single_output['data']['type']);
+    $first_include = reset($single_output['included']);
+    $this->assertEquals(
+      'user--user',
+      $first_include['type']
+    );
+    $last_include = end($single_output['included']);
+    $this->assertEquals(
+      'taxonomy_term--tags',
+      $last_include['type']
+    );
+    // 11. Includes with relationships.
+    $this->drupalGet('/jsonapi/node/article/' . $uuid . '/relationships/uid');
+    $single_output = Json::decode($this->drupalGet('/jsonapi/node/article/' . $uuid . '/relationships/uid', [
+      'query' => ['include' => 'uid'],
+    ]));
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertEquals('user--user', $single_output['data']['type']);
+    $this->assertArrayHasKey('related', $single_output['links']);
+    $this->assertArrayHasKey('included', $single_output);
+    $first_include = reset($single_output['included']);
+    $this->assertEquals(
+      'user--user',
+      $first_include['type']
+    );
+    $this->assertNotEmpty($first_include['attributes']);
+    $this->assertArrayNotHasKey('mail', $first_include['attributes']);
+    $this->assertArrayNotHasKey('pass', $first_include['attributes']);
+  }
+
+  /**
+   * Asserts that links are removed or present depending on the module setting.
+   *
+   * @param array $array
+   *   The array to check.
+   * @param bool $links_removed
+   *   TRUE if links attributes should be removed.
+   */
+  private function assertLinksRemoved(array $array, bool $links_removed): void {
+    if ($links_removed) {
+      $this->assertArrayNotHasKey('links', $array);
+    }
+    else {
+      $this->assertArrayHasKey('related', $array['links']);
+    }
+  }
+
+}