Alter Multiple Field Values by Testing "Field Type"
You can use FieldConfigInterface to get fields info.
Try the following:

use Drupal\Core\Field\FieldConfigInterface;

function MYMODULE_node_presave($node) {
  //- Call helper function to get all the fields
  $fieldsArray = getContentTypeFields($node-&gt;bundle());
  //- Loop through fields 
  foreach ($fieldsArray as $fieldName =&gt; $fieldConfig) {
    ///- Example to check if type is text_with_summary
    if ($fieldConfig-&gt;getType() == 'text_with_summary') {
      /**
       * @TODO : Something here for field of type text_with_summary
       */
    }
  }
}

/**
 * Helper function to Get all fields of content type:
 *
 * @param $contentType
 *
 * @return array
 */
function getContentTypeFields($contentType) {
  $entityManager = Drupal::service('entity.manager');
  $fields = [];
  if (!empty($contentType)) {
    $fields = array_filter(
      $entityManager-&gt;getFieldDefinitions('node', $contentType), function ($field_definition) {
      return $field_definition instanceof FieldConfigInterface;
    }
    );
  }

  return $fields;
}