default.twig

{% extends "base.html.twig" %}

{%  block javascripts %}
    <pre>{{  json }}</pre>
    <pre>{{  json | raw }}</pre>
    
    <script>
        var foo = {{ json | raw }}

        console.log(foo);

        alert(foo);
    </script>
{%  endblock %}

DefaultController.php

<?php

namespace App\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\Routing\Annotation\Route;

class DefaultController extends AbstractController
{
    /**
     * @Route("/", name="default")
     */
    public function index()
    {
        return $this->render('default.twig', ['json' => json_encode(['foo' => 'bar', 'bar' => 'foo'])]);
    }
}