How to retrieve all Variables from a Twig Template?

UPDATE 2019

Although {{ dump() }} does work, in some circumstances it may result in a "memory exhausted" error from PHP if it generates too much information (for example, due to recursion). In this case, try {{ dump(_context|keys) }} to get a list of the defined variables by name without dumping their contents.

UPDATE 2017

It is possible by using {{ dump() }} filter. Thanks for pointing that out in the comments!

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OUTDATED

It is not possible.

You can look for these variable in twig templates and add |default('your_value') filter to them. It will check if variable is defined and is not empty, and if no - will replace it with your value.

The way I do it is

<script>console.log({{ _context | json_encode | raw }});</script>

And then I just check my console using DevTools

This is useful I find to get all the top-level keys available in the current context:

<ol>
    {% for key, value in _context  %}
      <li>{{ key }}</li>
    {% endfor %}
</ol>

Thanks to https://www.drupal.org/node/1906780