Drupal - How add markup and prefix / suffix for computed field

I don't know where your variable $entity_field_item comes from, but you can generally add additional markup in at least three different ways:

1. Adding #prefix and #suffix to the fields render array using a preprocess function:

function mytheme_preprocess_field(&$variables, $hook) {
  $element = $variables['element'];
  if ($element['#field_name'] == 'field_myfield') {
    $variables['items'][0]['#prefix'] = '<div class="row">';
    $variables['items'][0]['#suffix'] = '<span class="suffix">Count</span></div>';
  }
}

2. Adding #prefix and #suffix in hook_node_view():

$node->content['field_myfield'][0]['#prefix'] = '<div class="row">';
$node->content['field_myfield'][0]['#suffix'] = '<span class="suffix">Count</span></div>';

3. Adding a new item to the $content array:

$node->content['my_custom_output'] = array(
  '#type' => 'markup',
  '#markup' => $entity_field_item['value'],
  '#prefix' => '<div class="row">',
  '#suffix' => '<span class="suffix">Count</span></div>',
);

Drupal 8

This example simply creates a new markup object from the original text value:

use Drupal\Core\Render\Markup;

/**
 * Implements template_preprocess_field__FIELD_NAME().
 */
function MYTHEME_preprocess_field__MYFIELD(&$variables) {

  $original_value = $variables['items'][0]['content']['#text'];
  $variables['items'][0]['content'] = Markup::create('<h1>' . $original_value . '</h1>');
}

You may want to check what's inside $variables['items'][0]['content'] first. As it absolutely depends on the field type and its formatter where the original value can be found:

• maybe $variables['items'][0]['content']['#text']
• maybe $variables['items'][0]['content']['#markup']
• maybe $variables['items'][0]['content']['#template']
• ...

---

This example simply adds some markup on top of the field output:

/**
 * Implements template_preprocess_field__FIELD_NAME().
 */
function MYTHEME_preprocess_field__MYFIELD(&$variables) {

  $variables['content']['#prefix'] = '<div class="linkbox-title"><span></span>' . t('Links') . '</div>';
}

---

This example wraps the output of every item of a multi-value field:

use Drupal\Core\Render\Element;

/**
 * Implements template_preprocess_field__FIELD_NAME().
 */
function MYTHEME_preprocess_field__MYFIELD(&$variables) {

  $element = $variables['element'];
  $items = Element::children($element);

  foreach ($items as $key) {
    $variables['items'][$key]['content']['#prefix'] = '<div class="MYCLASS item-' . $key . '">';
    $variables['items'][$key]['content']['#suffix'] = '</div>';
  };
}

If you are using a preprocess function (which would be my choice), you will want to account for the possibility of more than one item in the field:

  function mytheme_preprocess_field(&$variables) {
    if($variables['element']['#field_name'] == 'field_my_field'){
      foreach($variables['items'] as $key => $item){
        $variables['items'][ $key ]['#prefix'] = '<div class="row">';
        $variables['items'][ $key ]['#prefix'] = '<span class="suffix">Count</span></div>';
      }
    }
  }