Entity form display and Entity view display control which fields are shown in entity creation form and entity display page. Drupal provides complete control of fields and widgets through the user interface. We can also set these settings programmatically by loading correct configuration. 

    $settings = \Drupal::entityTypeManager()
      ->getStorage('entity_form_display')
      ->load('commerce_product.default.default')
      ->removeComponent('body')->save();

So when we run this snippet body field is removed from form display while creating a product of product type default. Feel free to experiment by setting other available configurations in configuration file of type entity view display and entity form display.  

You can use the following snippet in .install file to drop required tables when a module is uninstalled.

/**
 * Implements hook_uninstall()
 */
function your_custom_module_uninstall() {
  \Drupal::database()->schema()->dropTable('database_table_name');
}

Use the following function to log message to the terminal/command prompt from a Drush command.

$this->output()->writeln('Display this on terminal.');

Address field consists of various sub values which can be configured accordingly. To set an address field programmatically use the following snippet.

        $address = [
          'country_code' => 'IN',
          'address_line1' => 'Test Address Line 1',
          'address_line2' => 'Test Address Line 2',
          'locality' => 'City',
          'administrative_area' => 'New Delhi',
          'postal_code' => '110001',
        ]; 

$entity-set('field_address', $address);

$entity->save();

To restrict a route defined using routing.yml to the user having a certain role use the following snippet.

your_module.your_restricted_route:
  path: '/any-route'
  defaults:
    _form: '\Drupal\your_module\Form\RestrictedForm'
    _title: 'Restricted Settings'
  requirements:
    _role: 'role1+role2'

This route will only be accessible for user have 'role1' or 'role2'.

Most of the times the POST request endpoints are protected by basic authentication. Accessing them might give errors such as access denied or 500 internal server error. Here is post request snippet in Guzzle to handle requests in such cases.

try {
      $client = \Drupal::httpClient();
      $request = $client->post('/example-post-url/', [
        'auth' => ['username', 'password'],
        'json' => [
          'data' => $data,
        ],
      ]);
      $response = json_decode($request->getBody(), TRUE);
    }
    catch (RequestException $error) {
     $error_message =  $error->getMessage();

}

You can use the following snippet to use an internal link as a menu link.

your_module.your_internal_link:
  title: 'Internal Link'
  url: 'internal:/internal-link'
  parent: 'system.admin'
  description: 'An internal link added in the main admin menu for Drupal 8.'
  weight: -20

Reset button in form can be added directly in buildForm method of Form class or using a form alter.

$form['action']['reset'] = [

     '#type' => 'button',
      '#value' => 'Clear',
      '#attributes' => ['onclick' => 'this.form.reset(); return true;'],

];

You can use the following block code to load a form in a block in Drupal 8.

<?php

namespace Drupal\example_module\Plugin\Block;

use Drupal\Core\Block\BlockBase;

/**
 * Provides a 'ExmpleFormBlock' block.
 *
 * @Block(
 *  id = "ExampleFormBlock",
 *  admin_label = @Translation("Example Form Block"),
 * )
 */
class ExampleFormBlock extends BlockBase {

  /**
   * {@inheritdoc}
   */
  public function build() {
    return \Drupal::formBuilder()->getForm('Drupal\example_module\Form\ExampleForm');
  }

}
 

Example Form is the class of form which has to be displayed in the block. You can also use core and contributed provided forms.

Since comment post is a form, the submit button text can be changed via form alter the in following way.

/**
 * Implements hook_form_alter().
 */
function your_module_form_alter(&$form,  FormStateInterface $form_state, $form_id){
if($form_id == 'comment_comment_form') {
  $form['actions']['submit']['#value'] = t('POST A COMMENT');
  }
}