using {{ twig variables }} inside external jquery Developing a D8 theme, I am loading an external .js file containing some jQuery code (so it is not inside the TWIG template). 

Is it possible to access and modify TWIG variables like {{ myvariable }} inside the external jQuery?
It's not possible to access Twig variables in jQuery directly. Instead access the content of PHP variables inside of an external jQuery code by attaching them to drupalSettings. See the documentation:


  Attaching configurable JavaScript
  
  In some cases, you may want to add JavaScript to a page that depends
  on some computed PHP information.
  
  In this case, create a JavaScript file, define and attach a library
  just like before, but also attach JavaScript settings and have that
  JavaScript file read those settings, via drupalSettings (the successor
  to Drupal 7's Drupal.settings). However, to make drupalSettings
  available to our JavaScript file, we have to do the same work as we
  had to do to make jQuery available: we have to declare a dependency on
  it.

cuddly-slider:
  version: 1.x
  js:
    js/cuddly-slider.js: {}
  dependencies:
    - core/jquery
    - core/drupalSettings

  
  and

$build['#attached']['library'][] = 'fluffiness/cuddly-slider';
$build['#attached']['drupalSettings']['fluffiness']['cuddlySlider']['foo'] = 'bar';

  
  Where 'bar' is some calculated value.
  
  Then cuddly-slider.js will be able to access
  
  drupalSettings.fluffiness.cuddlySlider.foo (and it will === 'bar').


https://www.drupal.org/developing/api/8/assets