In DataWeave, functions and lambdas (anonymous functions) can be passed as values or be assigned to variables. 

DataWeave provides multiple ways to create functions. Just like we have named functions, we have functions without names, called lambdas. A lambda is a value in DataWeave, just like a String, an Object, and a Boolean. When using lambdas within the body of a DataWeave file in conjunction with a function such as map, its attributes can either be explicitly named or left anonymous, in which case they can be referenced as $, $$, etc.

The lambdas function syntax:

<function_name>(<arg1>, <arg2>, …, <argN>)

Examples:

DW Script

%dw 2.0
output json
---
(() -> 2 + 3)()

Output

5

You can define a function as a variable with a constant directive through var Input

{
  "field1": "Annie",
  "field2": "Point",
  "field3": "Stuff"
}

Transform

%dw 2.0
output application/json
var toUser = (user) -> {
  firstName: user.field1,
  lastName: user.field2
}
---
{
  "user" : toUser(payload)
}

Output

{
  "user": {
    "firstName": "Annie",
    "lastName": "Point"
  }
}

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