What is the core module?

dw::Core is imported automatically into every script.

What is the map placeholders?

$ = item, $$ = zero-based index.

What is the pluck placeholders?

$ = value, $$ = key, $$$ = index; returns an array.

What is the mapObject result?

An object; the mapper returns an object.

What is the reduce default accumulator?

First element; the lambda starts with the second.

What is the reduce on []?

null with no initial value; the initial value otherwise.

What is the groupBy result?

Object keyed by criteria values; each value an array of whole items.

Transforming Data10% of the exam

Core DataWeave collection functions

Predict what map, reduce, groupBy, pluck, mapObject, joinBy, flatten and distinctBy return, including the $, $$ and $$$ placeholders, reduce's default accumulator and the shape (array, object or string) each function produces.

Official exam objective

Predict the result of core DataWeave functions.

~7 min read · 7 practice questions · updated Oct 28, 2026

What the exam expects

Predict the output of map and pluck when the lambda uses the $, $$ and $$$ placeholders.
Trace reduce step by step with and without an initial accumulator value, including on an empty array.
Predict the shape groupBy returns (an object of arrays) and tell pluck (array) from mapObject (object).
Turn an array into a String with joinBy or reduce, and predict flatten and distinctBy on nested or duplicate data.
Key terms: lambda (anonymous function passed to a function such as (item, index) -> ...); placeholder ($, $$, $$$ used when the lambda's parameters are not named); accumulator (the running result reduce carries between iterations); criteria (the value a lambda returns that groupBy or distinctBy groups or de-duplicates on); dw::Core (the module of core functions every script imports automatically).

Key facts

Fact	Detail	Source
Core module	`dw::Core` is imported automatically into every script	Core (dw::Core) (opens in a new tab)
`map` placeholders	`$` = item, `$$` = zero-based index	map (opens in a new tab)
`pluck` placeholders	`$` = value, `$$` = key, `$$$` = index; returns an array	pluck (opens in a new tab)
`mapObject` result	An object; the mapper returns an object	mapObject (opens in a new tab)
`reduce` placeholders	`$` = item, `$$` = accumulator	reduce (opens in a new tab)
`reduce` default accumulator	First element; the lambda starts with the second	reduce (opens in a new tab)
`reduce` on `[]`	`null` with no initial value; the initial value otherwise	reduce (opens in a new tab)
`groupBy` result	Object keyed by criteria values; each value an array of whole items	groupBy (opens in a new tab)
`joinBy`	Array to a single String with the separator between items	joinBy (opens in a new tab)
`flatten` depth	One level only; empty subarrays omitted	flatten (opens in a new tab)
`distinctBy`	Lambda result is the uniqueness criteria; original items returned	distinctBy (opens in a new tab)

Core module

Detail: dw::Core is imported automatically into every script
Source: Core (dw::Core) (opens in a new tab)

map placeholders

Detail: $ = item, $$ = zero-based index
Source: map (opens in a new tab)

pluck placeholders

Detail: $ = value, $$ = key, $$$ = index; returns an array
Source: pluck (opens in a new tab)

mapObject result

Detail: An object; the mapper returns an object
Source: mapObject (opens in a new tab)

reduce placeholders

Detail: $ = item, $$ = accumulator
Source: reduce (opens in a new tab)

reduce default accumulator

Detail: First element; the lambda starts with the second
Source: reduce (opens in a new tab)

reduce on []

Detail: null with no initial value; the initial value otherwise
Source: reduce (opens in a new tab)

groupBy result

Detail: Object keyed by criteria values; each value an array of whole items
Source: groupBy (opens in a new tab)

joinBy

Detail: Array to a single String with the separator between items
Source: joinBy (opens in a new tab)

flatten depth

Detail: One level only; empty subarrays omitted
Source: flatten (opens in a new tab)

distinctBy

Detail: Lambda result is the uniqueness criteria; original items returned
Source: distinctBy (opens in a new tab)

Sources for this section (9)

Check yourself

What do the documented multiply examples return for [2,3,3] with no initial value and for [2,2,3] with acc = 3?
Why must a mapObject mapper return an object rather than a bare value?
When does reduce return null?
When should you choose pluck over mapObject?
When would a script need a second flatten?

The questions are free to read. Answers with their sources and self-grading are included with the pack.

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What the exam expects

Key facts

Check yourself

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