---
id: javascript
title: Building a JavaScript (Browser) Plugin
---
import useBaseUrl from '@docusaurus/useBaseUrl';
import Link from '@docusaurus/Link';

:::caution
This tutorial requires a browser that supports [WebSockets](https://developer.mozilla.org/en-US/docs/Web/API/WebSocket).
:::

This section of the tutorial goes through the three steps required to build a JavaScript plugin.

## Step 1 - install Flipper JavaScript SDK

Add the Flipper client to your web application. Run `npm install js-flipper` (`yarn add js-flipper`)

## Step 2 - start the Flipper client

:::warning
Do not start the Flipper client in production! Preferably, do not even include Flipper in your production builds!
:::

Use the following to start the Flipper client:

```tsx file=js/react-flipper-example/src/FlipperTicTacToe.tsx start=DOCS_START_CLIENT_START end=DOCS_START_CLIENT_END
```

## Step 3 - call `addPlugin` to add your plugin

To register a new plugin with Flipper, call `flipperClient.addPlugin` and pass your plugin as an object.

Your plugin must conform to the following interface:

```ts file=js/js-flipper/src/plugin.ts start=DOCS_FLIPPER_PLUGIN_START end=DOCS_FLIPPER_PLUGIN_END
```

The `onConnect` and `onDisconnect` events, featured in the above snippet, are triggered every time the plugin becomes (in)active in the Flipper desktop application.
If the plugin is a <Link to={useBaseUrl("/docs/extending/create-plugin#background-plugins")}>background plugin</Link>, these events are triggered typically only once (they might be triggered never if the Desktop user didn't enable the plugin, or multiple times if they enabled or disabled the plugin a few times).

The `onConnect` callback receive a `connection` which can be used to communicate with the backend:

```tsx file=js/react-flipper-example/src/FlipperTicTacToe.tsx start=DOCS_ADD_PLUGIN_START end=DOCS_ADD_PLUGIN_END
```

You might want to store the connection somewhere to be able to send more events as long as `onDisconnect` event hasn't been fired.

The `connection` object can also be used to listen to messages coming from the Desktop plugin. See <Link to={useBaseUrl("/docs/extending/create-plugin")}>Client Plugin API</Link> for details.

## Live demo

An example plugin to play a little Tic-Tac-Toe between the Flipper Desktop and a React app can be found inside this repository as well (run `yarn && yarn start` in `js/react-flipper-example` to start the test project):

 * The React plugin implementation - [FlipperTicTacToe.tsx](https://github.com/facebook/flipper/tree/main/js/react-flipper-example/src/FlipperTicTacToe.tsx)
 * The Flipper Desktop plugin implementation - [rn-tic-tac-toe/index.tsx](https://github.com/facebook/flipper/blob/main/desktop/plugins/public/rn-tic-tac-toe/index.tsx)