# Referencing flow variables in custom scripts

## Introduction

If required, you can reference (and therefore manipulate) flow variables in custom scripts. The possibilities here are only as limited as your development expertise however, a simple example might be where you want to generate a running count of order lines, to be output to a `total` field in a target system.

To achieve this, you would:

1. [Add a flow variable](https://doc.wearepatchworks.com/product-documentation/process-flows/building-process-flows/dynamic-variables/referencing-flow-variables-in-a-process-flow#stage-1-define-flow-variables-that-can-be-used) named `running_total` to your process flow settings.
2. [Write a custom script](https://doc.wearepatchworks.com/product-documentation/developer-hub/custom-scripting) which loops over each received order line and updates the `running_total` variable as it goes.
3. Add the custom script to your process flow via a [script shape](https://doc.wearepatchworks.com/product-documentation/process-flows/building-process-flows/process-flow-shapes/advanced-shapes/script-shape).
4. Add a [map shape](https://doc.wearepatchworks.com/product-documentation/process-flows/building-process-flows/process-flow-shapes/standard-shapes/map-shape) to your process flow and include a rule which maps a custom string transform for `{{flow.variables.running_total}}`, to the `total` field in the target system.

## Flow variable syntax for custom scripts

To reference flow variables in a custom script, the required syntax is as follows:

{% code title="PHP 8.1" lineNumbers="true" %}

```javascript
['flow']['variables']['variableName']
```

{% endcode %}

{% code title="Javascript Node 18" lineNumbers="true" %}

```javascript
flow.variables.variableName
```

{% endcode %}

{% code title="C# 8" lineNumbers="true" %}

```javascript
["flow"]["variables"]["variableName"]
```

{% endcode %}

{% code title="Python 3" lineNumbers="true" %}

```javascript
['flow']['variables']['variableName']
```

{% endcode %}

{% code title="Ruby 3" lineNumbers="true" %}

```javascript
['flow']['variables']['variableName']
```

{% endcode %}

In all cases, the `variableName` element should be replaced with the actual flow variable name. For example:

{% code title="PHP 8.1" lineNumbers="true" %}

```php
['flow']['variables']['running_total']
```

{% endcode %}

## Example

The example script below takes a **flow variable** named `customerID` and sets the value to `1234567`:

{% code title="PHP 8.1" lineNumbers="true" %}

```php
<?php

/**
 * Handler function.
 *
 * @param array $data [
 *      'payload'   => (string|null) the payload as a string|null
 *      'variables' => (array[string]string) any variables as key/value
 *      'meta'      => (array[string]string) any meta as key/value
 *    ]
 */

function handle($data)
{
    $data['flow']['variables']['customerID'] = 1234567;

    return $data;
}
```

{% endcode %}

So, wherever the `customerID` flow variable is referenced in a process flow, its value would be set to 1234567 when the process flow runs.

{% hint style="info" %}
When you update flow variables via a script, those updates persist for the duration of the flow run. Once the process flow has completed, default values are restored.
{% endhint %}