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  4. Flow Control in DataWeave

Flow Control and Scope Operations in DataWeave

You can use the following operators within any DataWeave expression:

Before you begin, note that DataWeave version 2 is for Mule 4 apps. For a Mule 3 app, refer to the DataWeave version 1 documentation set in the Mule 3.9 documentation. For other Mule versions, you can use the version selector for the Mule Runtime table of contents.

do

A do statement creates a scope in which new variables, functions, annotations, or namespaces can be declared and used. The syntax is similar to a mapping in that it is composed of a header and body separated by ---. Its header is where all the declarations are defined, and its body is the result of the expression.

This example uses do to return the string "DataWeave" when myfun() is called from the main body of the script.

DataWeave Script:
%dw 2.0
output application/json
fun myfun() = do {
    var name = "DataWeave"
    ---
    name
}
---
{ result: myfun() }

This example uses do to return the string "DataWeave" when the variable myVar is referenced from the main body of the script.

DataWeave Script:
%dw 2.0
output application/json
var myVar = do {
    var name = "DataWeave"
    ---
    name
}
---
{ result: myVar }

Both scripts produce this output:

Output:
{
  "result": "DataWeave"
}

The next example uses do to prepend the string "Foo" to a string (" Bar") that is passed to the test(p: String) function.

DataWeave Script:
%dw 2.0
output application/json
fun test(p: String) = do {
    var a = "Foo" ++ p
    ---
    a
}
---
{ result: test(" Bar") }
Output:
{
  "result": "Foo Bar"
}

See also, Examples: Local DataWeave Variables.

if else

An if statement evaluates a conditional expression and returns the value under the if only if the conditional expression returns true. Otherwise, it returns the expression under else. Every if expression must have a matching else expression.

The following example uses the input { country : "FRANCE" }, which is defined by the myVar variable in the header:

DataWeave Script:
%dw 2.0
var myVar = { country : "FRANCE" }
output application/json
---
if (myVar.country == "USA")
  { currency: "USD" }
else { currency: "EUR" }
Output:
{
  "currency": "EUR"
}

You can use the if-else construct on any condition that evaluates to true or false, including mathematical, logical, equality, and relational statements. The condition can act on any valid input.

The next DataWeave script applies if else statements to the result of the following conditional statements:

  • A mathematical operation in ex1
    The if else statement returns the Boolean value true if the operation 1 + 1 == 55 is true and false if not.

  • An equality operation in ex2
    The if else statement returns 1 if the value of the specified index is 1 or a string if the value is not 1.

  • An isEmpty function in ex3
    The if else statement returns the string "ID is empty" or "ID is not empty" depending on whether aRecord.bookId contains a value.

  • A mapping in ex4 that iterates over firstInput
    The if else statement returns the value of the bookId as a Number if the value is equal to 101. It returns the specified string if the value is not equal to 101.

DataWeave Script:
%dw 2.0
var aRecord =
 [
    "bookId":"101",
    "title":"world history",
    "price":"19.99"
 ]
output application/xml
---
{ examples:
	{
	   ex1 : if (1 + 1 == 55) true
	         else false,
	   ex2 : if ([1,2,3,4][1] == 1) 1
	         else "value of index 1 is not 1",
	   ex3 : if (isEmpty(aRecord.bookId)) "ID is empty"
	  	 else "ID is not empty",
	   ex4 : aRecord.bookId map (idValue) ->
	   	 if (idValue as Number == 101) idValue as Number
		 else "not 101"
	}
}
Output:
<?xml version='1.0' encoding='UTF-8'?>
<examples>
  <ex1>false</ex1>
  <ex2>value of index 1 is not 1</ex2>
  <ex3>ID is not empty</ex3>
  <ex4>101</ex4>
</examples>

Additional examples are available in DataWeave Operators.

else if

You can chain several else expressions together within an if-else construct by incorporating else if. The following example uses the input var myVar = { country : "UK" }, which is defined by the myVar variable in the header.

DataWeave Script:
%dw 2.0
var myVar = { country : "UK" }
output application/json
---
if (myVar.country =="USA")
	{ currency: "USD" }
else if (myVar.country =="UK")
	{ currency: "GBP" }
else { currency: "EUR" }
Output
{
  "currency": "GBP"
}

The following example is similar but takes an array as input instead of an object. The body of the script uses if else and else if statements within a do operation to populate the value of the hello variable.

DataWeave Script:
%dw 2.0
output application/json
---
["Argentina", "USA", "Brazil"] map (country) -> do {
  var hello = if(country == "Argentina") "Hola"
   else if(country == "USA") "Hello"
   else if(country == "Brazil") "Ola"
   else "Sorry! We don't know $(country)'s language."
   ---
   "$(hello) DataWeave"
}
Output:
[
  "Hola DataWeave",
  "Hello DataWeave",
  "Ola DataWeave"
]

See Also