New options will be added to $map, $filter and $reduce, allowing users to reference the array index of the element to which the operator's expression is currently being applied.

Description of Linked Ticket

Summary

Extend $map, $filter, $reduce to expose the index of array element.

Motivation

Sometimes when working with arrays the logic depends on the position of the element. E.g. two-dimentional array represents values on the grid, and element index is a coordinate on X/Y axis; or user may want to select every other element of the array to create a sample for A/B test or parallel processing.
It's pretty cumbersome to calculate position of the element today, so we'd like to introduce a simplified syntax for it.
Example from Paul's book

Documentation

Product Description
Scope
Syntax
Technical Design
Docs Update