Get The field values of node in Drupal 8

use Drupal\node\NodeInterface;

/**
 * Implements hook_ENTITY_TYPE_insert() for node entities.
 *
 * This tests saving a node on node insert.
 *
 * @see \Drupal\node\Tests\NodeSaveTest::testNodeSaveOnInsert()
 */
function node_test_node_insert(NodeInterface $node) {
  // Set the node title to the node ID and save.
  if ($node->getTitle() == 'new') {
    $node->setTitle('Node ' . $node->id());
    $node->setNewRevision(FALSE);
    $node->save();
  }
}

Now There is so many functions are there to get the values,
For All the functions available visit the API code,

https://api.drupal.org/api/drupal/core%21modules%21node%21src%21NodeInterface.php/interface/NodeInterface/8.2.x

Some of as below,

Node edit form, Drupal 8 Automatically Load the whole object no need to load the entity like below,

if ($event->getFormId() == 'node_alexa_audio_clips_edit_form') {
   $node = \Drupal::routeMatch()->getParameter('node');
   //file field value
   $node->get('field_audio')->getvalue()[0]['target_id'];

  or

  $node->field_audio->entity->getFileUri();
}

Load the Paragraph with in node directly:

$node = \Drupal::routeMatch()->getParameter('node');
            $para = $node->field_news_stream_content->referencedEntities();

//where field_news_stream_content is paragraph ref field

            foreach ($para as $node_para){
                $ref_entity = $node_para->entity;
                //kint($node_para->toArray());
            }

Get The node Field value

// Define usage at the top of your file.
use Drupal\node\Entity\Node;

//https://api.drupal.org/api/drupal/core%21modules%21node%21src%21Entity%21Node.php/class/Node/8.2.x
//it Will call the class node & can access all the functions of node,
$node = Node::load(25497);

let suppose we want to get the value of field
then use
$value = $node->get('field_issue_title')->value; // It will return the value if it has the val key in the field , like [nid][0][value],
if the value key is not present then use ,
$value = $node->get('field_issue_title')->getValue(); // if the structure is like [type][0][target_id], if use getvalue(), it will return the array [0][target_id] => 'test name'
$value = $node->get('field_issue_title')->getString(); // it direct return the value, test name

$node->toArray()// it return the whole array

We can load the node by Entity type query format also, like below

$entity = \Drupal::entityTypeManager()->getStorage('node')->load(25497);
print_r($entity->toArray());

http://www.drupal8.ovh/en/tutoriels/58/get-field-value-of-a-node-entity