Interview Questions & Answers DataWeave (2025)

 
 
Interview Questions & Answers  Dataweave (2025)

1. What is DataWeave and why is it used in MuleSoft?
Answer:
DataWeave is a MuleSoft transformation language (DW 2.x in Mule 4) designed to map, transform, enrich, and filter data across formats like JSON, XML, CSV, and Java objects. Typical use cases include API payload conversion, file transformation, and building integration pipelines.
 
2.Transform JSON to XML using DataWeave

Q: Write a DataWeave script that maps:
{ "name": "John", "age": 30, "email": "john@example.com" }
to:
<person>
  <name>John</name>
  <age>30</age>
  <email>john@example.com</email>
</person>
A:
%dw 2.0
output application/xml
---
{
  person: {
    name: payload.name,
    age: payload.age,
    email: payload.email
  }
}
 
3. Basic String Manipulation
Q: Convert "John Doe" to uppercase.
A:
%dw 2.0
output application/json
---
{ name: upper(payload.name) }
Transforms "John Doe" to {"name":"JOHN DOE"}
 
4. Filtering Arrays
Q: Filter items with value > 15.
A:
%dw 2.0
output application/json
---
payload filter ((item) -> item.value > 15)
Returns filtered objects
 
5. Combining First & Last Name
Q: Given {"firstName":"John","lastName":"Doe"}, combine into "fullName".
A:
%dw 2.0
output application/json
---
{ fullName: payload.firstName ++ " " ++ payload.lastName }
Yields {"fullName":"John Doe"}
 
6. Real-World Scenario: JSON→XML (CRM → Marketing Platform)
Scenario: Transform CRM JSON {firstName,lastName,email} into Marketing XML {GivenName,FamilyName,EmailAddress}.
Solution:
%dw 2.0
output application/xml
---
Customer: {
  GivenName: payload.firstName,
  FamilyName: payload.lastName,
  EmailAddress: payload.email
}
This real example enabled seamless CRM‑to‑platform integration
 
Real‑World DataWeave Scenarios & Solutions

A. Flatten Nested JSON to CSV
Scenario: Convert complex JSON to flat CSV.
Solution Outline:
·         Inspect nested payload structure.
·         Use flatten, map, pluck.
·         Convert to CSV via output application/csv
 
B. Date Parsing & Formatting
Scenario: Reformat "2023-01-01T00:00:00Z" to "01/01/2023".
Solution:
%dw 2.0
output application/json
---
payload as Date {format: "yyyy-MM-dd'T'HH:mm:ssz"}
      as String {format: "MM/dd/yyyy"}
Result: "01/01/2023"
 
C. Lookup Transformation
Use a map or file as lookup table; then map + find to enrich or merge data. Handle missing keys carefully
 
D. Grouping Data
Group transactions by customer ID using groupBy:
payload groupBy $.customerId
       mapObject ((customerId, txns) -> {
           customerId: customerId,
           total: sum(txns.*.amount)
       })
Computes total per customer
 
E. Handling Null Values
Strategies include:
·         default keyword
·         when or unless
·         flatten to remove nulls
Mention error handling implications
 
F. Performance with Large Files
·         Use streaming mode and batch jobs
·         Avoid in-memory ops
·         Employ caching for repeated lookups — essential for high‑volume integrations
 
Preparation Tips
·         Know core functions: map, filter, groupBy, flatten, joinBy, pluck, default, read/write
·         Practice scenarios: JSON⇄XML/CSV, lookup, aggregation
·         Master patterns: error handling, streaming, batch mode
·         Use tools: DataWeave Playground for quick testing
 
Sample Q&A Summary Table

Question	What to Explain	Key DW Concepts
Transform formats	Show correct input/output script	input/output directive
Handle null	Use default or remove nulls	default, when, flatten
Optimize large data	Streaming, batch, caching	streaming mode, batch job
Lookup enrich	Merge payload with reference	map, find, pluck
Group Data	Summarize by key	groupBy, sum