reduce()

Info

New in Atfinity 17.

Description

reduce() folds a list into a single value by threading an accumulator through the list, left to right. Unlike map() — which processes each element independently — reduce() lets each step build on the result of the previous one, so it can express running totals, products, and stateful checksums.

The arrow function takes two arguments: the accumulator and the current element. reduce() also takes an initial accumulator value, which is returned unchanged for an empty list.

Syntax

list.reduce(initial, (accumulator, current) => expression)

Returns: the final accumulator value.

Example

(1, 2, 3, 4, 5).reduce(0, (acc, x) => acc + x)

Returns 15 — the sum.

(1, 2, 3, 4, 5).reduce(1, (acc, x) => acc * x)

Returns 120 — the product.

('a', 'b', 'c').reduce('', (acc, x) => concat(acc, x))

Returns 'abc'.

Stateful checksums

reduce() can express checksums whose steps depend on the previous result — which map() cannot. For example, the ISO 7064 MOD 11,10 scheme (used by some national tax numbers) carries a running product across the digits:

digits.reduce(10, (product, digit) => (((digit + product - 1) % 10 + 1) * 2) % 11)