Create a field in a node entity programmatically on Drupal 8.

Example.

Step 1 : Create field storage.

\Drupal\field\Entity\FieldStorageConfig::create(array(

  'field_name' => 'field_text',

  'entity_type' => 'node',

  'type' => 'text',

  'cardinality' => -1,

))->save();

Step 2 : Attach an instance of the field to the page content type.

\Drupal\field\Entity\FieldConfig::create([

  'field_name' => 'field_text',

  'entity_type' => 'node',

  'bundle' => 'page',

  'label' => 'A Text field',

])->save();

Step 3 : Set From Display

entity_get_form_display('node', 'page', 'default')

  ->setComponent('field_text', array(

    'type' => 'text_textfield',

  ))

  ->save();

Step 4 : Set Display

entity_get_display('node', 'page', 'default')

  ->setComponent('field_text', array(

    'type' => 'text_default',

  ))

  ->save();

Tips

Load the field:

$field_config = \Drupal\field\Entity\FieldStorageConfig::loadByName('node', 'field_text');