How to display/debug variables in twig file Drupal 8?

Discovering and Inspecting Variables in Twig Templates using {{ dump() }} function.
  To dump only the available variable keys use: {{ dump(_context|keys) }}

  Uncomment the line that contains the twig_debug setting.php
  $settings['twig_debug'] = TRUE;

  Beware of dump()
  If you want to see all variables, but dump() results in exhausted memory because of recursion or the like, you can loop through _context to see all the keys in it:

    {% for key, value in _context  %}
      <li>{{ key }}</li>
    {% endfor %}
  Then use a conditional to check (for example {% if loop.index == 2 %}), and dump that value only if necessary.

   Devel has adopted a new library to print variables and it’s called Kint.
   kint(), ksm()