Executes a given reducer closure on every item of the list, accumulating to a single return value.

This function does not generate a change.


array.reduce(reducer, [initial])


Argument Type Description
reducer closure (required) Closure to execute on every value (except for the first, if no initial value is provided).
initial any (optional) The initial value serves as the first argument of the callback during the first call. If no initial value is provided, the first item in the array will be used instead and not processed by the callback. Calling reduce() on an empty list without an initial value will raise a lookup_err().

The reducer argument takes three optional arguments:

Argument Description
(0) accumulator An accumulated value that is returned by the previously invoked callback—or initial value.
(1) current The current item in the list that is being processed by the callback.
(2) index The index of the current item in the list that is being processed by the callback. If an initial value is given, it start from index 0, else, it starts from index 1.

Return value

The single value that results from the reduction.


Sum all the values of a list:

[0, 1, 2, 3].reduce(|a, b| a+b);

Return value in JSON format


Sum values in a thing list:

// An initial value is required
[{x: 1}, {x: 2}, {x: 3}].reduce(|a, b| a+b.x, 0);

Return value in JSON format


Flatten a list with tuples:

// Use an empty list as initial value
[[1, 2], [3, 4], [5, 6]].reduce(|a, b| {a.extend(b); a}, []);

Return value in JSON format

[1, 2, 3, 4, 5, 6]