> For the complete documentation index, see [llms.txt](https://dev.guardian.hedera.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://dev.guardian.hedera.com/guardian/standard-registry/policies/policy-creation/introduction/data-transformation-addon.md).
# Data Transformation Addon
The purpose of this block is to produce a transformed result based on the data from the source
and filters blocks.
This block can be used in conjunction with pagination and filters for the
source block.
## Usage
As shown in the example below, dataTransformationAddon blocks should be placed after
documentsSourceAddon and filtersAddon blocks.
## Properties
| Property Name | Description | Example |
| -------------------- | -------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Expression |
A custom JavaScript
function that transforms
the existing source data
and returns it in a
custom format
|
function main(documents){
let sum = 0;
for(let i=0; i< documents.length; i++) {
sum+=
documents\[i].document.credentialSubject\[0].fiel
d0 +
d1;
documents\[i].document.credentialSubject\[0].fiel
}
return {
sum
}
}
done(main(documents));
|
| hideWhenDiscontinued | Check if the button should be hidden when policy is discontinued | Checked/Unchecked |
## API
The API for the dataTransformationAddon uses the GET method
Example response to a GET request that returns the sum of 2 fields in a custom format:
```
{
"sum": 10
}
```
The request can be fulfilled through both endpoint
*`/api/v1/policies/{policyId}/blocks/{blockId}`*
and
*`/api/v1/policies/{policyId}/tag/{blockTag}/blocks`*