> ## Documentation Index > Fetch the complete documentation index at: https://docs.cassidyai.com/llms.txt > Use this file to discover all available pages before exploring further. # Run code > Execute custom JavaScript code within a Workflow to transform data, perform calculations, or implement custom logic. Execute custom JavaScript code within a [Workflow](/workflows/overview). Use this for data transformations, calculations, API calls, or custom logic that can't be handled by other actions. Your code runs in a secure, sandboxed environment. In the Workflow builder, click **+** between blocks and select **Run Code** from the action library. Action library with Run Code selected Adding the Run Code action to the Workflow Click **Advanced Settings** to access additional configuration. In the **NPM Packages** section, search and select any JavaScript libraries you need for your code. Dayjs (date formatting) and axios (HTTP requests) are included by default. NPM packages search and selection in Advanced Settings In the code editor, write your custom JavaScript code. [Reference variables](/guides/prompt-engineering#workflow-prompts) from previous steps or the trigger using **#** to use data from earlier in the Workflow. For managing secret API keys or other secure information, see [Secret Keys](/workflows/advanced/secret-keys). Code editor with variable references from previous steps When using NPM packages, include the import statement at the top of your code. Code editor showing NPM package import statement Optionally, click **Edit with Copilot** above the code editor to have AI write or modify code for you. Edit with Copilot button above the code editor Enter a prompt describing what you want the code to do. Copilot prompt describing the desired code change Submit your prompt and Copilot generates the code. Review the result and adjust as needed. Generated JavaScript code from the Copilot prompt The return value of your JavaScript code becomes the output of the action. In the **Output Fields** section, you have two options: * **Auto-detect from code** (recommended): When enabled, Cassidy analyzes the return statement in your code to automatically determine available outputs. These detected fields appear in the **Detected Fields** section and can be referenced in later Workflow steps. Auto-detect from code toggle enabled with detected output fields * **Define output fields manually**: Toggle off **Auto-detect from code** to manually add fields. Click **+ Add Field** to specify the field name and data type for each output value. For more details, see [Structured Output Fields](/workflows/advanced/structured-output-fields). Manually defined output fields with field name and data type ## Working with files in Run Code The Run Code action supports reading, creating, transforming, and outputting files using the built-in `CassidyFile` class and the `cassidy.files` helper methods. This lets you process file data, generate new files, download files from URLs, and pass files to subsequent Workflow steps. ### Inputting files To work with a file variable from a previous Workflow step, wrap the variable in the `CassidyFile` constructor: CassidyFile constructor wrapping a file variable from a previous step ### Working with files The `CassidyFile` class provides several convenience methods to get the content in different formats, including as a string, in base64, or as a buffer: CassidyFile class methods for reading file content as string, base64, or buffer The `cassidy.files` object also provides convenience methods for constructing and working with files: cassidy.files helper methods for creating and working with files ### Outputting files To output files from the action, return them in the output object. These will be available to use in later actions of the Workflow: Returning a file in the output object of a Run Code action File output available as a variable in subsequent Workflow steps The returned file can then be referenced as a variable in subsequent Workflow steps, such as Upload Knowledge Base File or any other action that accepts file inputs. For more information on creating files across Cassidy, see [How to create files in Cassidy](/guides/create-files). ## Related * [Build and configure Workflows](/workflows/building) * [Secret Keys](/workflows/advanced/secret-keys) * [Structured Output Fields](/workflows/advanced/structured-output-fields) * [How to create files in Cassidy](/guides/create-files)