Specifies a function similar to Array.prototype.reduce() to apply to each value within the slice or a configuration object for the built-in reducer, as in a ValuesSearch definition.
|$reducer||The reducing function.|
|$seed||The initial reduction passed to the reducing function.|
You can pass in either a configuration object for the built-in reducer or a function reference to a custom reducer. The built-in reducer configuration object can have the following properties:
- One of the string values "fragment", "item", or "none" (default). This setting controls whether or not to include the number of occurrences of each value. Fragment frequency is the number of fragments (documents) containing each value. Item frequency is the number occurrences of each value. Defaults to none.
When returning frequencies, the built-in reducer returns an
object with the serialized value (or bucket name when using
groupInto) as the key and the frequency as the value.
The reducer function should have the following signature:
function (prev, current, index, state)
Where the parameters have the following contents:
- The value returned by the previous call, or the seed value on the first call.
- The current value.
- The number of the current value.
- A state object with a boolean-valued
isLast> property that indicated the last call. You can set it to true to prematurely halt the reduction.
The return from the last call becomes the final reduced result.
You cannot use this method in conjunction with