Drupal 8

Install module: https://www.drupal.org/project/devel

// Example for brevity only, inject the request_stack service and call 
// getCurrentRequest() on it to get the request object if possible.
$request = \Drupal::request();
$is_ajax = $request->isXmlHttpRequest();

Test

/**
 * Implements hook_form_BASE_FORM_ID_alter().
 */
function MODULE_form_views_exposed_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  $storge = $form_state->getStorage();
  if (!empty($storge['view']) && $storge['view']->id() === 'my_view') {
    if (isset($form['actions']['reset']) && isset($form['actions']['submit'])) {
      $submit_id = $form['actions']['submit']['#id'];
      $form['actions']['reset']['#attributes']['onclick'] = 'javascript:jQuery(this.form).clearForm();jQuery("#' . $submit_id . '").trigger("click");return false;';
    }
  }
}

$config_factory = \Drupal::configFactory();
$langcode = $config_factory->get('system.site')->get('langcode');
$config_factory->getEditable('system.site')->set('default_langcode', $langcode)->save();

$bundle_label = \Drupal::entityTypeManager()
  ->getStorage('node_type')
  ->load($node->bundle())
  ->label();

$bundle_label = $node->type->entity->label();

<?php
/**
 * Debugging using the database connection.
 */
/** @var \Drupal\Core\Database\Connection */
$connection = \Drupal::service('database');
$query = $connection->select('node', 'node');
$query->fields('node', ['nid'])
  ->condition('node.type', 'page')
  
// Debug.
dump($query->__toString());

/**
 * Implements hook_query_QUERY_ID_alter().
 */
function mymodule_query_taxonomy_term_access_alter($query) {
  /** @var \Drupal\Core\Database\Query\Select $query */
  $conditions = &$query->conditions();
  foreach ($conditions as $i => $condition) {
    if (isset($condition['field']) && $condition['field'] === 't.default_langcode') {
      unset($conditions[$i]);
    }
  }
}

Disable cache for a custom page from route declaration.

If you want to disable cache for a custom controller (Custom module), You have no_cache option (YOUR_MODULE.routing.yml).
Example :
File : mymodule.routing.yml

system.admin:
  title: Administration
  route_name: system.admin
  weight: 9
  menu_name: admin
system.admin_content:

Update your THEMENAME.theme

How to get HTML from drupal 8 renderer array ?

How to convert Render array into HTML output code ?

$current_path = \Drupal::service('path.current')->getPath();
$front_uri = Drupal::config('system.site')->get('page.front');
$front_alias = \Drupal::service('path.alias_manager')->getAliasByPath($front_uri);
$current_alias = \Drupal::service('path.alias_manager')->getAliasByPath($current_path);
$isFrontPage = \Drupal::service('path.matcher')->isFrontPage() || $front_alias === $current_alias;

    $current_url = \Drupal\Core\Url::fromRoute('<current>', array(), array("absolute" => TRUE))->toString();

Open settings.php add these lines:

$settings['container_yamls'][] = DRUPAL_ROOT . '/sites/development.services.yml';
$settings['cache']['bins']['render'] = 'cache.backend.null';

Use drush with command drush cr to clear all cache