Check if field exists in a node
<?php
$entity->hasField('abc');
?>
Get value of a field:
<?php
use Drupal\node\Entity\Node
$title = Node::load($nid)->get('title')->value;
$node = \Drupal::entityManager()->getStorage('node')->load($nid); //Another way to load
?>
For User Added fields:
<?php
$terms = $node->get('abc')->getValue();
$terms = $node->abc->getValue();
?>
Get Nid from URL
<?php $nid = \Drupal::routeMatch()->getRawParameter($node_id); ?>
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Link
https://www.frobiovox.com/posts/2016/03/28/simplify-drupal-8-field-value-calls.html
https://lakshminp.com/using-entity-api-drupal-8