How to get tid on behalf of node id (nid) in Drupal 8?

Question

I am new in Drupal 8. I have created a node with category (taxonomy). Now I don't understand how to get tid by using nid. I am using $node->body->value to get the body field value and use {{ body }}. But to get category id if I use $node->field_category->value, it will display Null in debugging mode.

function THEME_preprocess_node(&$variables) {
  $node = \Drupal::routeMatch()->getParameter('node');
  $variables['body'] = $node->body->value;    // Working fine
  $cat_id = $node->field_category->value;    // Its showing null
  kint($cat_id);
}

So can anyone please tell me how can I get the category id (tid) of a node?

Answer 1

A reference field doesn't have a property value. You can use entity to load the term or target_id to get the term id:

$tid = $node->field_tags->target_id;

Answer 2

You can loop over the items like this:

foreach ($node->field_category as $item) {
  $tids[] = $item->target_id;
}

or get the entities like this:

$terms = $node->get('field_category')->referencedEntities();