{% set values = [1, 2] %}
{% set values = values | merge(['apple', 'orange']) %}
{# values now contains [1, 2, 'apple', 'orange'] #}
New values are added at the end of the existing ones.
The merge filter also works on hashes:
{% set items = { 'apple': 'fruit', 'orange': 'fruit', 'peugeot': 'unknown' } %}
{% set items = items | merge({ 'peugeot': 'car', 'renault': 'car' }) %}
{# items now contains { 'apple': 'fruit', 'orange': 'fruit', 'peugeot': 'car', 'renault': 'car' } #}
For hashes, the merging process occurs on the keys: if the key does not already exist, it is added but if the key already exists, its value is overridden.
If you want to ensure that some values are defined in an array (by given default values), reverse the two elements in the call:
{% set items = { 'apple': 'fruit', 'orange': 'fruit' } %}
{% set items = { 'apple': 'unknown' } | merge(items) %}
{# items now contains { 'apple': 'fruit', 'orange': 'fruit' } #}