Custom Twig Extension Sample for Drupal 8

acme_twig_helpers.services.yml

services:
  acme_twig_helpers:
    class: Drupal\acme_twig_helpers\TwigExtension
    tags:
      - { name: twig.extension }

TwigExtension.php

<?php

/* acme_twig_helpers/src/TwigExtension.php */

namespace Drupal\acme_twig_helpers;

use Drupal\Component\Utility\Html;
use Drupal\taxonomy\Entity\Term;
use Twig_Extension;
use Twig_SimpleFunction;

/**
 * Class TwigExtension.
 *
 * @package Drupal\acme_twig_helpers
 */
class TwigExtension extends Twig_Extension
{

  /**
   * {@inheritdoc}
   */
  public function getName(){
    return 'acme_twig_helpers';
  }

  /**
   * {@inheritdoc}
   */
  public function getFilters(){
    return [
      new \Twig_SimpleFilter('path_to_class', [$this, 'getPathToClass']),
    ];
  }

  /**
   * {@inheritdoc}
   */
  public function getFunctions(){
    return [
      new Twig_SimpleFunction('term_name', [$this, 'getTermName']),
    ];
  }

  /*
   * Converts '/path/like/this' to CSS class name.
   * Usefull in menu templates.
   *
   * Example: {{ '/about/contact' | path_to_class }}
   * Outputs: about-contact
   */
  public function getPathToClass($path) {
    $path = array_filter(explode('/', $path));
    $path = array_values($path);
    $path = implode('-', $path);
    return Html::getClass($path);
  }

  /*
   * Returns name of a term given its TID.
   * Might be useful in Views to override title
   * when using term contextual filters.
   *
   * Example: term_name(457)
   * Outputs: [name of the term 457]
   */
  public function getTermName($tid){
    // Normalize if $tid is a Markup
    $tid = "".$tid;
    $term = Term::load($tid);
    if($term) {
      return $term->getName();
    }
    return '';
  }

}