Drupal 8 Entity API cheat sheet

$nid = 123;     // example value
$node_storage = \Drupal::entityTypeManager()->getStorage('node');
$node = $node_storage->load($nid);

echo $node->id();  // 123

echo $node->bundle();    // 'article'
echo $node->getType();    // 'article' - this is the same as bundle() for most entities, but doesn't work for all entity types

echo $node->get('title')->value;           // "Lorem Ipsum..."
echo $node->get('created')->value;         // 1510948801
echo $node->get('body')->value;            // "The full node body, <strong>with HTML</strong>"
echo $node->get('body')->summary;          // "This is the summary"
// a custom text field
echo $node->get('field_foo')->value;       // "whatever is in your custom field"
// a file field
echo $node->get('field_image')->target_id; // 432 (a managed file FID)

echo $node->title->value;            // "Lorem Ipsum..."
echo $node->created->value;          // 1510948801
echo $node->body->value;             // "This is the full node body, <strong>with HTML</strong>"
echo $node->body->summary;           // "This is the summary"
echo $node->field_foo->value;        // "whatever is in your custom field"
echo $node->field_image->target_id;  // 432

$query = \Drupal::entityQuery('node')
  ->condition('type', 'article'),
  ->condition('field_foo', 42);
$nids = $query->execute();
$nodes = $node_storage->loadMultiple($nids);
 
foreach ($nodes as $n) {
  echo $n->title->value;            // "Lorem Ipsum..."
 
  // do whatever you would do with a node object (set fields, save, etc.)
  $n->set('title', "this is a test");
  $n->save();
}

$node->set('title', "Moby Dick");
$node->set('body', array(
'summary' => "Book about a whale",
'value' => "Call me Ishmael...",
'format' => 'basic_html',
));
$node->save();

// one
$nid = 42;      // example value
$node = $node_storage->load($nid);
$node->delete();
 
// multiple
$nids = [21,12,45,67];  // example value
$nodes = $node_storage->loadMultiple($nids);
$node_storage->delete($nodes);
 
// multiple, loading one at a time to avoid "out of memory" errors - may be slow
$nids = [21,12,45,67];  // example value
foreach($nids as $nid)
{
  $node = $node_storage->load($nid);
  $node->delete();
}

$my_paragraph = null;
 
foreach ($node->get('field_paragraph_reference') as $para) {
  if ($para->entity->getType() == 'your_paragraph_type') {   // e.g., "main_content" or "social_media"
    $my_paragraph = $para->entity;
  }
}
 
if (!empty($my_paragraph)) {
  // $my_paragraph is a regular entity and can be interacted with like any other entity
  echo $my_paragraph->field_somefield->value;
 
  // (however, they don't have a "title" like a node)
  echo $my_paragraph->title->value;  // <-- this won't work
} else {
  echo "The node doesn't have this paragraph type.";
}

echo $my_paragraph->getType(); // "main_content"

$fid = 42;      // example value
$file_storage = \Drupal::entityTypeManager()->getStorage('file');
$file = $file_storage->load($fid);

$file = $node->field_image->entity;

echo $file->getFileUri();   // "public://file123.jpg"
// if you want the URL without Drupal's custom scheme, you can translate it to a plain URL:
echo file_url_transform_relative(file_create_url($file->getFileUri()));   // "/sites/default/files/public/file123.jpg"
echo $file->filename->value;   // "file123.jpg"
echo $file->filemime->value;   // "image/jpeg"
echo $file->filesize->value;   // 63518  (size in bytes)
echo $file->created->value;    // 1511206249  (Unix timestamp)
echo $file->changed->value;    // 1511234256  (Unix timestamp)
echo $file->id();              // 432

echo $file->uid->target_id;               // 1
echo $file->uid->value;                   // <-- doesn't work. Use target_id instead.
echo $file->uid->entity->name->value;     // "alice"
echo $file->uid->entity->timezone->value; // "America/Los_Angeles"

foreach ($node->field_my_entity_reference as $reference) {
 
  // if you chose "Entity ID" as the display mode for the entity reference field,
  // the target_id is the ONLY value you will have access to
  echo $reference->target_id;    // 1 (a node's nid)
 
  // if you chose "Rendered Entity" as the display mode, you'll be able to 
  // access the rest of the node's data.
  echo $reference->entity->title->value;    // "Moby Dick"
 
}

$nids = [3,4,5,6];   // example value
$node->set('field_my_entity_reference', $nids);
$node->save();

$nids = [3,4,5,6];   // example value
foreach ($nids as $nid) {
  $node->field_my_entity_reference[] = [
    'target_id' => $nid
  ];
}
$node->save();