Add sum filter to standard filters

[adamklingbaum]

A new filter to sum the values of an array. Either an ordinary sum for an array of numeric values, or as the sum of property values for an array of objects.

{{ array | sum }}

or

{{ array | sum: {property} }}

Example usage

{{ card_balances | sum }} 

or

{{ articles | sum: 'comments_count' }} 

[charlespwd]

LGTM! :)