---
schema_version: "1"
id: 4599a1c68fe06720afea56f78ab4068edd0c5709d0d9c86491db6e559bcef055
title: Use Constant Directives
collection: mulesoft
version: current
locale: en-us
portal_url: https://docs.salesforce.com/mulesoft/current/en-us/dataweave/2.3/dataweave-cookbook-use-constant-directives.md
canonical_url: https://docs.mulesoft.com/dataweave/2.3/dataweave-cookbook-use-constant-directives
source_site: docs.mulesoft.com
content_type: documentation
content_hash: bff3ad69d7f14878fe24c30522439021f8f4a71a010dad717e97d0316fd00009
description: MuleSoft Documentation Site
release: "2.3"
latest: false
---
---
product: DataWeave
version: 2.3
keywords: studio, anypoint, transform, transformer, format, aggregate, rename, xml, json, metadata, dataweave, data weave, datamapper, dwl, dfl, dw, output structure, input structure, map, mapping, concatenate,
---

# Use Constant Directives

> For the full documentation index, see: https://docs.mulesoft.com/llms.txt

This DataWeave example builds URLs from constants that are defined as variables (`var`) in the
DataWeave header. It also conditionally outputs fields if they are present in the input.

The example uses these functions:

- `++` (concatenate) to build constant directives into URLs.

- `map` to go through the fields in the input.

- `if` to output `summary` and `brand` fields.

DataWeave Script:

```dataweave
%dw 2.0
var baseUrl = "http://alainn-cosmetics.cloudhub.io/api/v1.0"
var urlPage = "http://alainn-cosmetics.cloudhub.io/api/v1.0/items"
var fullUrl = "http://alainn-cosmetics.cloudhub.io/api/v1.0/items"
var pageIndex = 0
var requestedPageSize = 4
output application/json
---
using (pageSize = payload.getItemsResponse.PageInfo.pageSize) {
     links: [
        {
            href: fullUrl,
            rel : "self"
        },
        {
            href: urlPage ++ "?pageIndex=" ++ (pageIndex + pageSize) ++ "&pageSize=" ++ requestedPageSize,
            rel: "next"
        },
        ({
            href: urlPage ++ "?pageIndex=" ++ (pageIndex - pageSize) ++ "&pageSize=" ++ requestedPageSize,
            rel: "prev"
        }) if(pageIndex > 0)
     ],
     collection: {
        size: pageSize,
        items: payload.getItemsResponse.*Item map (item) -> {
            id: item.id,
            'type': item.'type',
            name: item.name,
            (summary: item.summary) if(item.summary?),
            (brand: item.brand) if(item.brand?),
            links: (item.images.*image map (image) -> {
                href: trim(image),
                rel: image.@'type'
            }) + {
                href: baseUrl ++ "/" ++ item.id,
                rel: "self"
            }
        }
     }
}
```

Input XML Payload:

```xml
<ns0:getItemsResponse xmlns:ns0="http://www.alainn.com/SOA/message/1.0">
    <ns0:PageInfo>
        <pageIndex>0</pageIndex>
        <pageSize>5</pageSize>
    </ns0:PageInfo>
    <ns1:Item xmlns:ns1="http://www.alainn.com/SOA/model/1.0">
        <id>B0015BYNRO</id>
        <type>Oils</type>
        <name>Now Foods LANOLIN PURE</name>
        <images>
            <image type="SwatchImage">http://ecx.images-amazon.com/images/I/11Qoe774Q4L._SL30_.jpg
            </image>
        </images>
    </ns1:Item>
    <ns1:Item xmlns:ns1="http://www.alainn.com/SOA/model/1.0">
        <id>B002K8AD02</id>
        <type>Bubble Bath</type>
        <name>Deep Steep Honey Bubble Bath</name>
        <summary>Disclaimer: This website is for informational purposes only.
            Always check the actual product label in your possession for the most
            accurate ingredient information due to product changes or upgrades
            that may not yet be reflected on our web site. These statements made
            in this website have not been evaluated by the Food and Drug
            Administration. The products offered are not intended to diagnose,
            treat
        </summary>
        <images>
            <image type="SwatchImage">http://ecx.images-amazon.com/images/I/216ytnMOeXL._SL30_.jpg
            </image>
        </images>
    </ns1:Item>
    <ns1:Item xmlns:ns1="http://www.alainn.com/SOA/model/1.0">
        <id>B000I206JK</id>
        <type>Oils</type>
        <name>Now Foods Castor Oil</name>
        <summary>One of the finest natural skin emollients available</summary>
        <images>
            <image type="SwatchImage">http://ecx.images-amazon.com/images/I/21Yz8q-yQoL._SL30_.jpg
            </image>
        </images>
    </ns1:Item>
    <ns1:Item xmlns:ns1="http://www.alainn.com/SOA/model/1.0">
        <id>B003Y5XF2S</id>
        <type>Chemical Hair Dyes</type>
        <name>Manic Panic Semi-Permanent Color Cream</name>
        <summary>Ready to use, no mixing required</summary>
        <images>
            <image type="SwatchImage">http://ecx.images-amazon.com/images/I/51A2FuX27dL._SL30_.jpg
            </image>
        </images>
    </ns1:Item>
    <ns1:Item xmlns:ns1="http://www.alainn.com/SOA/model/1.0">
        <id>B0016BELU2</id>
        <type>Chemical Hair Dyes</type>
        <name>Herbatint Herbatint Permanent Chestnut (4n)</name>
        <images>
            <image type="SwatchImage">http://ecx.images-amazon.com/images/I/21woUiM0BdL._SL30_.jpg
            </image>
        </images>
    </ns1:Item>
</ns0:getItemsResponse>
```

Output JSON:

```json
{
  "links": [
    {
      "href": "http://alainn-cosmetics.cloudhub.io/api/v1.0/items",
      "rel": "self"
    },
    {
      "href": "http://alainn-cosmetics.cloudhub.io/api/v1.0/items?pageIndex=5&pageSize=4",
      "rel": "next"
    }
  ],
  "collection": {
    "size": "5",
    "items": [
      {
        "id": "B0015BYNRO",
        "type": "Oils",
        "name": "Now Foods LANOLIN PURE",
        "links": [
          {
            "href": "http://ecx.images-amazon.com/images/I/11Qoe774Q4L._SL30_.jpg",
            "rel": "SwatchImage"
          },
          {
            "href": "http://alainn-cosmetics.cloudhub.io/api/v1.0/B0015BYNRO",
            "rel": "self"
          }
        ]
      },
      {
        "id": "B002K8AD02",
        "type": "Bubble Bath",
        "name": "Deep Steep Honey Bubble Bath",
        "summary": "Disclaimer: This website is for informational purposes only.\n            Always check the actual product label in your possession for the most\n            accurate ingredient information due to product changes or upgrades\n            that may not yet be reflected on our web site. These statements made\n            in this website have not been evaluated by the Food and Drug\n            Administration. The products offered are not intended to diagnose,\n            treat\n        ",
        "links": [
          {
            "href": "http://ecx.images-amazon.com/images/I/216ytnMOeXL._SL30_.jpg",
            "rel": "SwatchImage"
          },
          {
            "href": "http://alainn-cosmetics.cloudhub.io/api/v1.0/B002K8AD02",
            "rel": "self"
          }
        ]
      },
      {
        "id": "B000I206JK",
        "type": "Oils",
        "name": "Now Foods Castor Oil",
        "summary": "One of the finest natural skin emollients available",
        "links": [
          {
            "href": "http://ecx.images-amazon.com/images/I/21Yz8q-yQoL._SL30_.jpg",
            "rel": "SwatchImage"
          },
          {
            "href": "http://alainn-cosmetics.cloudhub.io/api/v1.0/B000I206JK",
            "rel": "self"
          }
        ]
      },
      {
        "id": "B003Y5XF2S",
        "type": "Chemical Hair Dyes",
        "name": "Manic Panic Semi-Permanent Color Cream",
        "summary": "Ready to use, no mixing required",
        "links": [
          {
            "href": "http://ecx.images-amazon.com/images/I/51A2FuX27dL._SL30_.jpg",
            "rel": "SwatchImage"
          },
          {
            "href": "http://alainn-cosmetics.cloudhub.io/api/v1.0/B003Y5XF2S",
            "rel": "self"
          }
        ]
      },
      {
        "id": "B0016BELU2",
        "type": "Chemical Hair Dyes",
        "name": "Herbatint Herbatint Permanent Chestnut (4n)",
        "links": [
          {
            "href": "http://ecx.images-amazon.com/images/I/21woUiM0BdL._SL30_.jpg",
            "rel": "SwatchImage"
          },
          {
            "href": "http://alainn-cosmetics.cloudhub.io/api/v1.0/B0016BELU2",
            "rel": "self"
          }
        ]
      }
    ]
  }
}
```

## Related Examples

- [Define a Custom Addition Function](https://docs.mulesoft.com/dataweave/2.3/dataweave-cookbook-define-a-custom-addition-function)

- [Conditionally Reduce a List through a Function](https://docs.mulesoft.com/dataweave/2.3/dataweave-cookbook-conditional-list-reduction-via-function)

## See Also

- [DataWeave Selectors](https://docs.mulesoft.com/dataweave/2.3/dataweave-selectors)

- [DataWeave Cookbook](https://docs.mulesoft.com/dataweave/2.3/dataweave-cookbook)