3.21String concatenation

Suppose:

{
  "firstName": "John",
  "lastName": "Smith"
}

You want:

John Smith

DataWeave concatenation uses:

payload.firstName ++ " " ++ payload.lastName

For example:

{
    Full_Name__c:
        payload.firstName ++ " " ++ payload.lastName
}

Remember:

++

You'll see it frequently.