Module.php

<?php

namespace modules;

use Craft;

class Module extends \yii\base\Module
{
    /**
     * Initializes the module.
     */
    public function init()
    {
        // Set a @modules alias pointed to the modules/ directory
        Craft::setAlias('@modules', __DIR__);

        // Set the controllerNamespace based on whether this is a console or web request
        if (Craft::$app->getRequest()->getIsConsoleRequest()) {
            $this->controllerNamespace = 'modules\\console\\controllers';
        } else {
            $this->controllerNamespace = 'modules\\controllers';
        }

        parent::init();

        // Custom initialization code goes here...
        Craft::$app->getView()->registerTwigExtension(new TwigExtension());
    }
}

TwigExtension.php

<?php

namespace modules;

use craft\helpers\StringHelper;
use yii\helpers\Inflector;

class TwigExtension extends \Twig_Extension implements \Twig_Extension_GlobalsInterface
{
    public function getFunctions()
    {
        return [
            new \Twig_Function('foo', [$this, 'fooFunction']),
        ];
    }

    public function fooFunction($value)
    {
        return 'FOO';
    }

    public function getFilters()
    {
        return [
            new \Twig_Filter('safeTruncate', [StringHelper::class, 'safeTruncate']),
            new \Twig_Filter('camel2words', [Inflector::class, 'camel2words']),
            new \Twig_Filter('json_decode', 'json_decode'),

            new \Twig_Filter('bar', function() { return 'BAR'; }),
        ];
    }

    public function getGlobals()
    {
        return [
            'foo' => 'bar',
        ];
    }
}