render_classes

The render_classes function can convert an array into a string suited for html class-attribute.

{{ render_classes(['class1', 'class2']) }}

{# outputs "class1 class2" #}

It ignores empty spaces:

{{ render_classes(['class1',' ', '', 'class2']) }}

{# outputs "class1 class2" #}

It also ignores values not considered suitable for classes:

{{ render_classes(false, 'class1', 'class2', null) }}

{# outputs "class1 class2" #}