filter

The filter filter filters elements of a sequence or a mapping using an arrow function. The arrow function receives the value of the sequence or mapping:

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{% set sizes = [34, 36, 38, 40, 42] %}

{{ sizes|filter(v => v > 38)|join(', ') }}
{# output 40, 42 #}

Combined with the for tag, it allows to filter the items to iterate over:

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{% for v in sizes|filter(v => v > 38) -%}
    {{ v }}
{% endfor %}
{# output 40 42 #}

It also works with mappings:

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{% set sizes = {
    xs: 34,
    s:  36,
    m:  38,
    l:  40,
    xl: 42,
} %}

{% for k, v in sizes|filter(v => v > 38) -%}
    {{ k }} = {{ v }}
{% endfor %}
{# output l = 40 xl = 42 #}

The arrow function also receives the key as a second argument:

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{% for k, v in sizes|filter((v, k) => v > 38 and k != "xl") -%}
    {{ k }} = {{ v }}
{% endfor %}
{# output l = 40 #}

Note that the arrow function has access to the current context.