Computed field example for Drupal 8/9

BlogAuthorComputed.php

<?php
// in MY_MODULE/src/ComputedField/BlogAuthorComputed.php


namespace Drupal\MY_MODULE\ComputedField;

use Drupal\Core\Field\EntityReferenceFieldItemList;
use Drupal\Core\TypedData\ComputedItemListTrait;

class BlogAuthorComputed extends EntityReferenceFieldItemList {
  use ComputedItemListTrait;

  protected function computeValue() {
    $delta = 0;
    $entity = $this->getEntity();

    $this->list[$delta] = $this->createItem($delta, $entity->getOwner()->id());
  }
}

my_module.php

<?php

/**
 * Implements hook_entity_bundle_field_info().
 */
function MY_MODULE_entity_bundle_field_info(EntityTypeInterface $entity_type, $bundle, array $base_field_definitions) {
  $new_definitions = [];

  if ($entity_type->id() === 'node') {

    if ($bundle == 'blog') {
      $new_definitions['blog_author'] = \Drupal\Core\Field\BaseFieldDefinition::create('entity_reference')
        ->setLabel(t('Author (computed)'))
        ->setComputed(TRUE)
        ->setClass('\Drupal\MY_MODULE\ComputedField\BlogAuthorComputed')
        ->setSettings(array(
          'target_type' => 'user',
          'default_value' => 0,
          ))
          ->setDisplayConfigurable('view', TRUE);
    }
  }
  return $new_definitions;
}