\Drupal::messenger()->addMessage('This is a regular message');
\Drupal::messenger()->addStatus('This is a status message, meaning status is OK or successful (green).');
\Drupal::messenger()->addError('This is an error message (red)');
\Drupal::messenger()->addWarning('This is a warning message (orange)');

namespace Drupal\my_module\Plugin\Block;

use Drupal\Core\Block\BlockBase;
use Drupal\Core\Block\BlockPluginInterface;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Session\AccountInterface;
use Drupal\Core\Access\AccessResult;
use Drupal\Core\Cache\UncacheableDependencyTrait;

/**
 * Provides a 'SetTask' Block.
 *
 * @Block(
 *   id = "dashboard_newtask_block",
 *   admin_label = @Translation("Dashboard New Task"),
 *   category = @Translation("DrupalVIP"),
 * )
 */
class NewTaskBlock extends BlockBase implements BlockPluginInterface {
    use UncacheableDependencyTrait;
    
    /**
    * {@inheritdoc}
    */
    public function build() {
        // Do NOT cache a page with this block on it.
        \Drupal::service('page_cache_kill_switch')->trigger(); 
        // build from form               
        $build = \Drupal::formBuilder()->getForm('Drupal\drupalvip_dashboard\Form\NewTaskForm');
        $build['#attributes']['class'][] = 'my_class';
        $build['#cache']['max-age'] = 0;
        $build['#cache']['contexts'] = [];
        $build['#cache']['tags'] = [];                   
        return $build;
    }       
  
    /**
    * {@inheritdoc}
    */
    public function blockForm($form, FormStateInterface $form_state) {
        $form = parent::blockForm($form, $form_state);
        $config = $this->getConfiguration();
        $form['block_note'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Note'),
            '#description' => $this->t('block note '),
            '#default_value' => isset($config['block_note']) ? $config['block_note'] : '',
        ];
        return $form;
    }  

    /**
    * {@inheritdoc}
    */
    public function blockSubmit($form, FormStateInterface $form_state) {
        parent::blockSubmit($form, $form_state);        
        $values = $form_state->getValues();
        $this->configuration['block_note'] = $values['block_note'];
    }    
    
} // end of class

<?php

namespace Drupal\basic_module\Plugin\Block;

use Drupal\Core\Block\BlockBase;

/**
 * Block annotation
 * 
 * @Block(
 *      id = "basic_block",
 *      admin_label = @Translation("Basic Block"),
 * )
 */
class BasicBlock extends BlockBase {
   /**
     * {@inheritDoc}
     */
    public function build() {
        $markup = '';
        $markup .= '<div>';
        $markup .= '    <p>Anything can come here: code, variables, html, etc.  </p>' ;
        $markup .= '</div>';
        return [
            '#type' => 'markup',
            '#markup' => $markup,
        ];
    }    
    
} // end of class

public function submitForm(array &$form, FormStateInterface $form_state) {
  $this->logger('user')->notice('Deleted %ip', ['%ip' => $this->banIp,]);
  $this->messenger()->addStatus($this->t('The IP address %ip was deleted.', [    '%ip' => $this->banIp,  ]));
  $form_state->setRedirectUrl($this->getCancelUrl());
}

        public function requestResidenceUpload(Request $request): AjaxResponse {
            $arg = $request->get('arg1');
            
            // Add an AJAX command to open a modal dialog with the form as the content.      
            $response = new AjaxResponse();
            $modal_form = $this->formBuilder->getForm('Drupal\mymodule\Form\MyForm', $arg);
            $response->addCommand(new OpenModalDialogCommand('Upload', $modal_form, ['width' => 600, 'height'=>600]) );
            return $response;        
        } 

import React from "react";

function Course(props) {
  return (
    <div>
      <ul>
        <li>{props.courseName}</li>
      </ul>
    </div>
  )
}

function App() {
  return (
    <div>
      <h1>Guvi Courses</h1>
      <Course courseName="Full Stack Development"  />
    </div>
  );
}

export default App;

import React, { useEffect, useState } from "react"

const UsingFetch = () => {
  const [users, setUsers] = useState([])

  const fetchData = () => {
    fetch("https://jsonplaceholder.typicode.com/users")
      .then(response => {
        return response.json()
      })
      .then(data => {
        setUsers(data)
      })
  }

  useEffect(() => {
    fetchData()
  }, [])

  return (
    <div>
      {users.length > 0 && (
        <ul>
          {users.map(user => (
            <li key={user.id}>{user.name}</li>
          ))}
        </ul>
      )}
    </div>
  )
}

export default UsingFetch

import React, { useEffect, useState } from "react";

function handleClick() {
    alert('You clicked node ');
}

export default function RWTaskIssues({nodeid}) {        
    const FetchAPI = '/rwtask/issues?task=' + nodeid + '&op=get';
    console.log("api fetch: " + FetchAPI);
    
    const [issues, setIssues] = useState([]);
    const fetchData = () => {
        fetch(FetchAPI)
            .then(response => {
                return response.json();
            })
            .then(response => {
                console.log("response data: " + response.data.items);
                setIssues(response.data.items);
            });
    };

    useEffect( () => {
        fetchData();
    }, []) ;   
    
    console.log("issues data: " + issues);
    
    return (
        <div className="block block-layout-builder block-field-blocknoderwtaskfield-rwtask-issue">
            <h2 className="block-title">תיאור הבעיה</h2>
            <div className="block-content">               
                {issues.length > 0 && (
                    <div className="field field-name-field-rwtask-issue field-type-text-long field-label-hidden field-items">
                        {issues.map(item => (
                            <div key={item.key} className="field-item" dangerouslySetInnerHTML={{__html: item.value}} />
                        ) ) }
                    </div>
                )}
                <button onClick={handleClick}>
                    Edit
                </button>            
            </div>
        </div>
    );        
}

var ajaxSettings = {
  url: '/node/1',
  dialogType: 'modal',
  dialog: { width: 400 },
};
var myAjaxObject = Drupal.ajax(ajaxSettings);
myAjaxObject.execute();

use Symfony\Component\HttpFoundation\JsonResponse;

/**
 * Class JsonApiArticlesController
 * @package Drupal\mymodule\Controller
 */
class JsonApiArticlesController {

  /**
   * @return JsonResponse
   */
  public function index() {
    return new JsonResponse([ 'data' => $this->getData(), 'method' => 'GET', 'status'=> 200]);
  }

  /**
   * @return array
   */
  public function getData() {

    $result=[];
    $query = \Drupal::entityQuery('node')
      ->condition('type', 'article')
      ->sort('title', 'DESC');
    $nodes_ids = $query->execute();
    if ($nodes_ids) {
      foreach ($nodes_ids as $node_id) {
        $node = \Drupal\node\Entity\Node::load($node_id);
        $result[] = [
          "id" => $node->id(),
          "title" => $node->getTitle(),
        ];
      }
    }
    return $result;
  }
}

use Symfony\Component\HttpFoundation\JsonResponse;

// if you know the data to send when creating the response
$response = new JsonResponse(['data' => 123]);

// if you don't know the data to send or if you want to customize the encoding options
$response = new JsonResponse();
// ...
// configure any custom encoding options (if needed, it must be called before "setData()")
//$response->setEncodingOptions(JsonResponse::DEFAULT_ENCODING_OPTIONS | \JSON_PRESERVE_ZERO_FRACTION);
$response->setData(['data' => 123]);

// if the data to send is already encoded in JSON
$response = JsonResponse::fromJsonString('{ "data": 123 }');

RESTful Example using jQuery and core REST module

CREATE Item

var package = {}
package.title = [{'value':'t1'}]
package.body = [{'value':'b1'}]
package._links = {"type":{"href":"http://local.drupal8.org/rest/type/node/page"}}
$.ajax({
  url: "http://example.com/entity/node",
  method: "POST",
  data: JSON.stringify(package),
  headers: {
    "Accept": "application/json",
    "Content-Type": "application/hal+json"
  },
  success: function(data, status, xhr) {
    debugger
  }
})

GET Item

$.ajax({
  url: "http://example.com/node/3?_format=hal_json",
  method: "GET",
  headers: {
    "Content-Type": "application/hal+json"
  },
  success: function(data, status, xhr) {
    debugger
  }
})

GET an Item and then UPDATE Item

$.ajax({
  url: "http://example.com/node/3?_format=hal_json",
  method: "GET",
  headers: {
    "Content-Type": "application/hal+json"
  },
  success: function(data, status, xhr) {
    var package = {}
    package.title = data.title
    package.body = data.body
    package.title[0].value = 'yar'
    package._links = {"type":{"href":"http://example.com/rest/type/node/page"}}
    debugger

    $.ajax({
      url: "http://example.com/node/3",
      method: "PATCH",
      data: JSON.stringify(package),
      headers: {
        "X-CSRF-Token": "niCxgd5ZZG25YepbYtckCy7Q2_GL2SvMUY5PINxRAHw",
        "Accept": "application/json",
        "Content-Type": "application/hal+json"
      },
      success: function(data, status, xhr) {
        debugger
      }
    })
  }
})

Drupal protects its REST resources from CSRF attacks by requiring a X-CSRF-Token request header to be sent when using a non-safe method. So, when performing non-read-only requests, that token is required. 
Such a token can be retrieved at /session/token.

GET an Item and then UPDATE Item with CSRF token.

$.ajax({
  url: 'http://example.com/session/token',
  method: 'GET',
  success: function(token) {
    var csrfToken = token;

    $.ajax({
      url: "http://example.com/node/3?_format=hal_json",
      method: "GET",
      headers: {
        "Content-Type": "application/hal+json"
      },
      success: function(data, status, xhr) {
        var package = {};
        package.title = data.title;
        package.body = data.body;
        package.title[0].value = 'yar';
        package._links = {"type":{"href":"http://example.com/rest/type/node/page"}};
        debugger

        $.ajax({
          url: "http://example.com/node/3",
          method: "PATCH",
          data: JSON.stringify(package),
          headers: {
            "X-CSRF-Token": csrfToken,
            "Accept": "application/json",
            "Content-Type": "application/hal+json"
          },
          success: function(data, status, xhr) {
            debugger;
          }
        });
      }
    });
  }
});

DELETE Item

$.ajax({
  url: "http://example.com/node/3",
  method: "DELETE",
  headers: {
    "Accept": "application/json",
    "Content-Type": "application/hal+json"
  },
  success: function(data, status, xhr) {
    debugger
  }
})

function mymodule_cron() {

  // Short-running operation example, not using a queue:
  // Delete all expired records since the last cron run.
  $expires = \Drupal::state()->get('my_module.last_check', 0);
  $request_time = \Drupal::time()->getRequestTime();
  \Drupal::database()
    ->delete('my_module_table')
    ->condition('expires', $expires, '>=')
    ->execute();
  \Drupal::state()->set('my_module.last_check', $request_time);

  // Long-running operation example, leveraging a queue:
  // Queue news feeds for updates once their refresh interval has elapsed.
  $queue = \Drupal::queue('my_module.feeds');
  $ids = \Drupal::entityTypeManager()
    ->getStorage('my_module_feed')
    ->getFeedIdsToRefresh();
  foreach (Feed::loadMultiple($ids) as $feed) {
    if ($queue->createItem($feed)) {

      // Add timestamp to avoid queueing item more than once.
      $feed->setQueuedTime($request_time);
      $feed->save();
    }
  }
  $ids = \Drupal::entityQuery('my_module_feed')
    ->accessCheck(FALSE)
    ->condition('queued', $request_time - 3600 * 6, '<')
    ->execute();
  if ($ids) {
    $feeds = Feed::loadMultiple($ids);
    foreach ($feeds as $feed) {
      $feed->setQueuedTime(0);
      $feed->save();
    }
  }
}

    public function buildForm(array $form, FormStateInterface $form_state, $dashboard = 0, $project=0, $sprint=0, $redirect='') {  //
      $config = \Drupal::config('drupalvip_task.settings');
      $log = $config->get('log');
      ($log)? \Drupal::logger("drupalvip_task")->debug(__FUNCTION__ . ": START "):'';      
      ($log)? \Drupal::logger("drupalvip_task")->debug("Form Args: da[$dashboard] pr[$project] sp[$sprint] rd[$redirect]"):'';
      
      $form['redirect'] = array(
          '#type' => 'value',
          '#value' => $redirect,
      );
      
      //dashboard select
      $dashboard_default = ($dashboard==0) ? null : $dashboard;
      $dashboard_options = DashboardController::getDashboards();
      $form['dashboard_select'] = [
        '#type' => 'select',
        '#title' => t('Dashboard'),
        //'#name' => 'dashboard-select',  <-- prevent the form_state to update
        '#options' => $dashboard_options,
        '#sort_options' => TRUE,
        '#required' => TRUE,
        '#empty_option' => "-- SELECT --",
        '#size' => 1,
        '#default_value' => $dashboard_default,
        '#attributes' => [
          'class' => ['use-ajax'],
          //'id' => "edit-dashboard-select", <-- this prevent the callback
        ],         
        '#ajax' => [
          'callback' => '::submitFormDashboardSelected',
          'wrapper' => 'group-select-wrapper',
          'suppress_required_fields_validation' => TRUE,
        ],        
      ];      
      
      $form['group'] = [
        '#type' => 'fieldgroup',
        '#prefix' => '<div id="group-select-wrapper">',
        '#suffix' => '</div>',         
      ];      
        $options[0] = "-- select --";        
        $form['group']['project_select'] = [
          '#type' => 'select',
          '#title' => $this->t('Project'),
          '#options' => $options,
          '#limit_validation_errors' => [],
          //'#empty_option' => "-- select --",
          '#size' => 1,        
          '#default_value' => 0, 
        ]; 
        if ($dashboard > 0) {
          $project_options = DashboardController::getProjects($dashboard);
          $project_options_2 = $options + $project_options ;
          $form['group']['project_select']['#options'] = $project_options_2; //$project_options; 
          $form['group']['project_select']['#default_value'] = $project;
        }      

        $form['group']['sprint_select'] = [
          '#type' => 'select',
          '#title' => $this->t('Sprint'),
          '#options' => $options,
          '#limit_validation_errors' => [],
          //'#empty_option' => "-- select --",
          '#size' => 1,        
          '#default_value' => 0, 
        ]; 
        if ($dashboard > 0) {
          $sprint_options = DashboardController::getSprints($dashboard);
          $sprint_options_2 = $options + $sprint_options ;
          $form['group']['sprint_select']['#options'] = $sprint_options_2; 
          $form['group']['sprint_select']['#default_value'] = $sprint;
        }      
        
      $form['subject'] =  [
        '#type' => 'textfield',
        '#title' => $this->t('Subject'),
        '#required' => TRUE,
        '#attributes' => [
          //'id' => "task-subject-box",
        ],        
      ];
      
      $form['actions'] = [
        '#type' => 'actions',
        '#attributes' => [
          'class' => ['form_actions'],
        ],         
      ];
        $form['actions']['cancel'] = [
          '#type' => 'submit',
          '#value' => $this->t('Cancel'),
          '#attributes' => [
            'class' => ['button','use-ajax','form-submit-modal','action-cancel' ],
          ],
          '#ajax' => [
            'callback' => [$this, 'submitFormClose'],
            'event' => 'click',
          ],
        ];       
        $form['actions']['create'] = [
          '#type' => 'submit',
          '#value' => $this->t('Create'),
          '#attributes' => [
            'class' => ['button','use-ajax','form-submit-modal','action-create' ],
          ],
          '#ajax' => [
            'callback' => [$this, 'submitFormCreate'],
            'event' => 'click',
            'onclick' => 'javascript:var s=this;setTimeout(function(){s.value="Saving...";s.disabled=true;},1);',
          ],
        ];              

      $form['#attributes']['class'][] = 'drupalvip_form';  
      $form['#attached']['library'][] = 'drupalvip_task/content';
      $form['#attached']['library'][] = 'core/drupal.dialog.ajax';
      $form['#attached']['library'][] = 'core/drupal.ajax';
      $form['#attached']['library'][] = 'core/jquery.form';
      
      ($log)? \Drupal::logger("drupalvip_task")->debug(__FUNCTION__ . ": RETURN FORM "):'';
      return $form;    
    }

<?php
  namespace Drupal\drupalvip\Form;

  use Drupal\Core\Form\FormBase;
  use Drupal\Core\Form\FormStateInterface;
  
  class AjaxForm extends FormBase {    

    public function getFormId() {
      return 'mymodule_ajax_form';
    }

    public function buildForm(array $form, FormStateInterface $form_state) {
      $triggering_element = $form_state->getTriggeringElement();

      $form['country'] = [
        '#type' => 'select',
        '#title' => $this->t('Country'),
        '#options' => [
          'serbia' => $this->t('Serbia'),
          'usa' => $this->t('USA'),
          'italy' => $this->t('Italy'),
          'france' => $this->t('France'),
          'germany' => $this->t('Germany'),
        ],
        '#ajax' => [
          'callback' => [$this, 'reloadCity'],
          'event' => 'change',
          'wrapper' => 'city-field-wrapper',
        ],
      ];

      $form['city'] = [
        '#type' => 'select',
        '#title' => $this->t('City'),
        '#options' => [
          'belgrade' => $this->t('Belgrade'),
          'washington' => $this->t('Washington'),
          'rome' => $this->t('Rome'),
          'paris' => $this->t('Paris'),
          'berlin' => $this->t('Berlin'),
        ],
        '#prefix' => '<div id="city-field-wrapper">',
        '#suffix' => '</div>',
        '#value' => empty($triggering_element) ? 'belgrade' : $this->getCityForCountry($triggering_element['#value']),
      ];

      return $form;
    }

    public function reloadCity(array $form, FormStateInterface $form_state) {
      return $form['city'];
    }

    protected function getCityForCountry($country) {
      $map = [
        'serbia' => 'belgrade',
        'usa' => 'washington',
        'italy' => 'rome',
        'france' => 'paris',
        'germany' => 'berlin',
      ];
      return $map[$country] ?? NULL;
    }

    public function submitForm(array &$form, FormStateInterface $form_state) {  }

} // end of class

$response = new AjaxResponse(); 
$response->addCommand(new ReplaceCommand('#edit-date-format-suffix', '<small id="edit-date-format-suffix">' . $format . '</small>')); 
return $response;

'#ajax' => [
  'callback' => 'Drupal\config_translation\FormElement\DateFormat::ajaxSample',
  'event' => 'keyup',  
  'progress' => [ 'type' => 'throbber', 'message' => NULL,  ], 
],