> For the complete documentation index, see [llms.txt](https://docs.fastn.ai/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.fastn.ai/flow-setup-essentials/designing-a-flow/custom-code.md).
# Custom Code
Write your own logic inside the flow using supported programming languages such as **JavaScript, Python Lambda, or C#**. This step is useful when you need more control, advanced data handling, or custom logic that cannot be achieved with standard components.
**Example (C# Custom Code)**
For example, if you need to add a **Custom Code** step in your flow, you can write a function that processes the data coming from previous steps and returns the output you want to use later. In the example below, the function is written in **C#**.
It takes the flow data as input, extracts the `steps` object (which contains outputs from earlier steps), deserializes it into a dictionary, and then returns it. This allows you to handle the step data programmatically inside your flow and make it available for the next steps.
> This example shows how you can configure the **Custom Code** step in **C#**, but the same component also works with **JavaScript** or **Python Lambda**, depending on your preference.
{% hint style="info" %}
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{% endhint %}
---
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