# Building a new extension

You’ve installed the CLI and created a Hello World extension—let’s dive into building your own custom extension!

This guide assumes you’ve already completed [Getting started](https://airtable.com/developers/extensions/guides/getting-started.md) and the
[Hello world tutorial](https://airtable.com/developers/extensions/guides/hello-world-tutorial.md). If you’re looking to get more familiar with the platform but
don’t have a extension in mind to build yet, try the [To-do list tutorial](https://airtable.com/developers/extensions/guides/to-do-list-tutorial.md)—it walks through
building a simple extension from the ground up.

## React in Extensions

Extensions are [React](https://reactjs.org) applications, with Airtable-specific utilities available to you via the Blocks
SDK. After you `block init`, you can build your extension as you would a standard React extension. You can import and use the
Blocks SDK just like any other npm package.

If you’re not familiar with React, we recommend taking a look at their
[Getting Started](https://react.dev/learn) guide. The core React concepts used in the Blocks SDK
are [function components](https://react.dev/learn/describing-the-ui) and
[hooks](https://react.dev/reference/react/hooks). (Our hooks are also compatible with class components—see our note
[here](https://airtable.com/developers/extensions/guides/read-data-from-airtable.md#using-hooks-with-class-components).)

## Starting from an existing extension

When creating a new extension, you can start from any of our existing [example extensions](https://airtable.com/developers/extensions/examples).
These provide a working extension for you to jump in and customize.

Here are some good starting points for different use cases:

-   **Custom embeds:** the [URL preview](https://github.com/Airtable/blocks-url-preview) extension shows an embedded preview
    when a cell with a URL is selected. (You can also install this extension from the extensions gallery!)
-   **Custom reports:** the [Print records](https://github.com/Airtable/blocks-print-records) extension reads record data from
    your base and formats it for printing.
-   **Custom charts:** the [Simple chart](https://github.com/Airtable/blocks-simple-chart) extension reads record data from
    your base and graphs it using [Chart.js](https://www.chartjs.org/).
-   **API integrations:** the [Wikipedia enrichment](https://github.com/Airtable/blocks-wikipedia-enrichment) extension
    fetches data from an external API and saves it back to the base
-   **Updating records:** the [Update records](https://github.com/Airtable/blocks-update-records) extension updates selected
    records when a button is clicked

You can also remix an open source extension if you have a link to the GitHub repo.

If none of the examples catch your eye or you just prefer a blank slate, create your extension using the “Hello world”
template. This template is available in both JavaScript and [TypeScript](https://www.typescriptlang.org).

## Useful features of the Blocks SDK

The [API reference](https://airtable.com/developers/extensions/api) has full details, but here are some key features for inspiration:

-   You can fetch data for tables, views, fields, and records, and the extension will live update as that data
    changes—see the [Read data from Airtable](https://airtable.com/developers/extensions/guides/read-data-from-airtable.md) guide for details.
-   You can create, update, and delete records—see the [Write back to Airtable](https://airtable.com/developers/extensions/guides/write-back-to-airtable.md) guide
    for details.
-   You can see what table and view the user has open, and what records or fields they have selected with the
    [Cursor](https://airtable.com/developers/extensions/api/models/cursor.md) API.
-   You can quickly build extensions that match the Airtable UI with a library of pre-built UI components. The library
    includes components such as [Button](https://airtable.com/developers/extensions/api/ui/components/button.md), [CellRenderer](https://airtable.com/developers/extensions/api/ui/components/cellrenderer.md),
    [TablePickerSynced](https://airtable.com/developers/extensions/api/ui/components/tablepickersynced.md), and many more.

## Useful resources

-   The [Custom Extensions community forum](https://community.airtable.com/c/custom-blocks-beta/54) is the best place for asking
    questions about building extensions. You can also see what other developers have built and share feedback.
-   When you’ve finished your extension, the [Polishing and sharing your extension guide](https://airtable.com/developers/extensions/guides/polishing-your-app.md) has a
    checklist for getting your extension production ready for others to use.
-   More to come in the future! Let us know in the
    [community forum](https://community.airtable.com/c/custom-blocks-beta/54) if there are specific resources that would
    be helpful.