Array Processing with JEXL Script

Process arrays with for loops, aggregations, and conditional filtering using multi-line JEXL Script support in the Message Builder policy.

Overview

The Message Builder policy supports both single-line JEXL expressions and multi-line JEXL scripts. Script mode is activated when the template expression contains var, for, while, or if/else constructs.

Key Differences

Feature	JEXL Expression	JEXL Script
Single-line evaluation	✅	✅
Multi-line code	❌	✅
for loop	❌	✅
var declaration	❌	✅
if/else block	❌	✅
return statement	❌	✅

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Example 1: Converting a Tag Array to a JSON Array

Input Body

{
  "tags": [
    { "id": 1, "name": "outdoor" },
    { "id": 2, "name": "premium" },
    { "id": 3, "name": "sale" }
  ]
}

Template (JEXL Script)

#{
  var result = '[';
  var first = true;
  for (tag : body.$.tags) {
    if (!first) { result = result + ','; }
    result = result + '"' + tag.name + '"';
    first = false;
  }
  result + ']'
}

Body variables extracted via JSONPath are automatically converted to Java List or Map objects, so they support .size() and index access.

Expected Output

["outdoor","premium","sale"]

---

Example 2: Summing Order Items

Input Body

{
  "items": [
    { "name": "Book", "price": 29.99, "quantity": 2 },
    { "name": "Pen", "price": 4.50, "quantity": 5 },
    { "name": "Notebook", "price": 12.00, "quantity": 3 }
  ]
}

Template (JEXL Script)

#{
  var total = 0;
  for (item : body.$.items) {
    total = total + (item.price * item.quantity);
  }
  total
}

Expected Output

118.48

---

Example 3: Conditional Array Filtering

Filter array elements that meet a specific condition and build a new array.

Input Body

{
  "products": [
    { "name": "Laptop", "stock": 0 },
    { "name": "Phone", "stock": 15 },
    { "name": "Tablet", "stock": 0 },
    { "name": "Monitor", "stock": 8 }
  ]
}

Template (JEXL Script)

#{
  var result = '[';
  var first = true;
  for (p : body.$.products) {
    if (p.stock > 0) {
      if (!first) { result = result + ','; }
      result = result + '"' + p.name + '"';
      first = false;
    }
  }
  result + ']'
}

Expected Output

["Phone","Monitor"]

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Example 4: Role-Based Authorization with if/else

Assign an authorization level based on user roles.

Setup

Assume a custom variable userRoles is already populated (e.g., from the request body) with value ["admin","editor"].

Template (JEXL Script)

#{
  var roles = userRoles;
  var level = 'NONE';
  for (var role : roles) {
    if (role == 'admin') {
      level = 'FULL';
    } else if (role == 'editor' && level != 'FULL') {
      level = 'WRITE';
    } else if (role == 'viewer' && level == 'NONE') {
      level = 'READ';
    }
  }
  level
}

Result

Roles	authLevel Value
["admin"]	FULL
["editor","viewer"]	WRITE
["viewer"]	READ
[]	NONE

---

Example 5: Accessing Array Elements with $[n] Syntax

To access a specific element of a root-level array, use $[n] syntax.

Input Body (Root Array)

[
  { "id": 1, "name": "First" },
  { "id": 2, "name": "Second" }
]

Variable Reference in Template

#{body.$[0].name}

Output

First

$[0] refers to the first element of the root array. $[1].id would return the id of the second element.

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Tips and Common Mistakes

How do I reference body array variables?

Use the standard body.$.path syntax directly inside the JEXL script. For example, for (tag : body.$.tags) iterates over the tags array. JSON array and object values are automatically converted to Java List and Map objects, enabling iteration and property access.

Can I use the `return` keyword?

Yes, in JEXL Script mode you can use return value explicitly. However, returning a value from the last expression (as in examples above) is the recommended approach.

Can I include `{` and `}` inside script templates?

Yes. The Message Builder uses a balanced brace scanner to correctly identify #{} blocks, including those with nested curly braces such as JSON object literals.

Can I use while loops?

Yes, JEXL Script supports while loops as well:

#{
  var list = body.$.items;
  var i = 0;
  var sum = 0;
  while (i < list.size()) {
    sum = sum + list[i].value;
    i = i + 1;
  }
  sum
}