DataWeave 2.1 is compatible with Mule 4.1. Standard Support for Mule 4.1 ended on November 2, 2020, and this version of Mule will reach its End of Life on November 2, 2022, when Extended Support ends.

Deployments of new applications to CloudHub that use this version of Mule are no longer allowed. Only in-place updates to applications are permitted.

MuleSoft recommends that you upgrade to the latest version of Mule 4 that is in Standard Support so that your applications run with the latest fixes and security enhancements.

countBy(Array<T>, (T) -> Boolean): Number

Counts the elements in a list (array) that match the results of a function.


Name Description


The input array that contains elements to match.


A function to apply to elements in the input array.


This counts the values in the array that are equal to the result of the matchingFunction ((($ mod 2) == 0)).


 %dw 2.0
 import * from dw::core::Arrays
 output application/json
{ "countBy" : [1, 2, 3, 4] countBy (($ mod 2) == 0) }


{ "countBy": 2 }

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