# Transform data and reassign variables

import { Steps } from "@astrojs/starlight/components"

A **variable** holds a named value that later steps in the same flow can read. You declare starting values under flow
**Variables**, then use **Transform data** steps to create new variables or **reassign** an existing name as the run
progresses. Screenshots on this page build a Google search URL by appending host and path onto a shared `url`
variable.

## When to use

- You need a named value that several steps read, or that you update as the flow runs (for example a growing URL,
  counter, or filtered list).
- You reshape or calculate data between steps without calling an external system.
- Prefer [Platform Artifacts](../store-data-with-platform-artifacts/) when the payload must persist across runs or be
  shared by other flows in the same environment.
- Prefer a [transform function](../transform-functions/) reference page when you already know which formula to apply
  and only need syntax and examples.

Before you begin, open a flow in the designer with permission to edit it.

## Guide

### Declare global variables

Flow **Variables** are the starting values for the run. Studio labels this panel **Global variables**. Any variable you
declare here is available to later steps, and a **Transform data** step can reassign the same name.

<ol>
<li>

**Open flow settings.** In the designer toolbar, choose the settings (gear) control.

</li>
<li>

**Open Variables.** Select the **Variables** tab.

</li>
<li>

**Add a variable.** Choose **Add item**, set the name (for example `url`), then set the initial value (plain text,
JSON, a `/` data pick, or a formula).
<div class="docs-screenshot-compact">

![Flow Variables tab showing Global variables callout and url set to https://](./assets/flow-global-variables.png)

</div>

</li>
</ol>
### Add a Transform data step

**Transform data** is a built-in flow step. Use it to create variables, reassign existing names, and apply formula
functions between other steps.

1. **Open Add a step.** On the canvas, choose **+** where the transform should run.
2. **Choose Transform data.** Under **Flow**, select **Transform data**. Studio places a purple **Transform data**
   node on the canvas.

### Assign and reassign variables

On a **Transform data** step, the **Variables** tab lists name and value rows—the same key/value form as global
variables. To **create** a variable, add a row with a new name. To **reassign**, add a row whose name matches an
existing global or upstream variable; the new value replaces the previous one for later steps.

You can chain several assignments in one step. Later rows may reference earlier ones in the same step (for example
append text onto the value you just set).

<ol>
<li>

**Open Variables on the step.** Select the **Transform data** step, then open the **Variables** tab.

</li>
<li>

**Add or edit a row.** Choose **Add item**. Set the variable name. For a reassignment, use the same name as the
global or earlier variable (for example `url`).

</li>
<li>

**Set the value.** Type a literal, combine text with a reference, or map data from earlier in the flow. Chain rows in
order when each assignment builds on the previous value of the same name.
<div class="docs-screenshot-compact">

![Transform data Variables tab with three url rows: literal https://, pill for URL www.google.com, then url pill plus /search](./assets/transform-data-variables-tab-three-url.png)

</div>

</li>
<li>

**Pick an existing variable with /.** In a value field, type `/` to open the data picker, then choose **Variables** and
select the variable whose current value you want to insert as a pill. That inserts a reference into the value; it does
not change the row name.
<div class="docs-screenshot-compact">

![Transform data step with slash data picker open on Variables, url selected](./assets/transform-data-step-slash-data-picker.png)

</div>

</li>
</ol>
### Apply transform functions

Each variable value can use formula functions (the **fx** control on the field). Chain functions with **Add a new
function** when you need hashes, formatting, array helpers, or other transforms. For parameter lists and examples, see
[Transform functions](../transform-functions/).

## Example

An operator builds a Google search URL in one **Transform data** step by reassigning `url` three times:

1. **Start the base.** Set `url` to `https://`.
2. **Append the host.** Reassign `url` to the previous value plus `www.google.com` (via a `/` pick or typed text).
3. **Append the path.** Reassign `url` again to the previous value plus `/search`, so later steps receive
   `https://www.google.com/search`.

## Keep in mind

- Reassignment is by **name match** only. There is no separate Reassign control—use the same variable name on a later
  **Transform data** row or step.
- Variables are scoped to the **current flow run**. They are not shared across environments or stored like Platform
  Artifacts.
- After a run, open the step **Logs** tab to inspect computed variable values when debugging.

## Related

[Work with data](../) · [Transform functions](../transform-functions/) ·
[Store data with Platform Artifacts](../store-data-with-platform-artifacts/)