Strict

Strict is based on the Stark theme, but will not throw errors if strict_variables is TRUE (unlike all core themes).

#1806538: Remove @todo about enabling strict_variables in Twig resolved that strict_variables should remain at its default value of FALSE. Since then core themes have gotten sloppy to the point of incorrectly documenting which variables are optional and which are required. #2445705: Core themes should not throw exceptions when strict_variables is TRUE was an attempt to correct this in core, but was rejected for a number of reasons.

What are strict variables and why use them

In a Twig template, if a variable or attribute does not exist you will receive a null value when the strict_variables option is set to false; alternatively, if strict_variables is true, Twig will throw an error. This is useful for finding typos or copy/paste errors early in the dev cycle.

In addition, Drupal best practices recommend turning on error_reporting (in PHP) and adding "use strict" to Javascript files. Seems we should be able to do it in Twig templates as well!

How do I turn on strict_variables?

Add the following line in your sites/[site_name|default]/services.yml file just following the twig.config: line (approx line 40):

strict_variables: true