Executes a given reducer closure on each element of the list, resulting in a single output value.

This function does not generate an event.


list.reduce(reducer, [initial])


Argument Type Description
reducer closure (required) Closure to execute on each value (except for the first, if no initial value is supplied).
initial any (optional) A value to use as the first argument to the first call of the callback. If no initial value is supplied, the first element in the array will be used and skipped. Calling reduce() on an empty array without an initial value will raise a lookup_err().

The reducer argument takes three optional arguments:

Argument Description
(0) accumulator The accumulator accumulates callback’s return values. It is the accumulated value previously returned in the last invocation of the callback—or initialValue.
(1) current The current element being processed in the list.
(2) index The index of the current element being processed in the array. Starts from index 0 if an initial value is provided. Otherwise, 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]