Working in Twig, I had to add classes to a pre-existing array.  Whilst I've grown used to the patterns in PHP, these cannot be applied in Twig.  Instead I had to merge the new array elements. Non associative arrays Working from a base level, I'll begin as if the original array hasn't been set.  The merge filter expects as first argument an array whose content will be merged with the assigned variable: {% set myArray = [] %} {% set myArray = myArray|merge(['tw-flex-inline'])...