reduce

The reduce filter iteratively reduces a sequence or a mapping to a single value using an arrow function, so as to reduce it to a single value. The arrow function receives the return value of the previous iteration and the current value and key of the sequence or mapping:

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{% set numbers = [1, 2, 3] %}

{{ numbers|reduce((carry, v, k) => carry + v * k) }}
{# output 8 #}

The reduce filter takes an initial value as a second argument:

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{{ numbers|reduce((carry, v, k) => carry + v * k, 10) }}
{# output 18 #}

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