# 
React Scan
React Scan automatically detects performance issues in your React app.
- Requires no code changes -- just drop it in
- Highlights exactly the components you need to optimize
- Always accessible through a toolbar on page
### Quick Start
```bash
npx -y react-scan@latest init
```
### [**Try out a demo! →**](https://react-scan.million.dev)
## Install
The `init` command will automatically detect your framework, install `react-scan` via npm, and set up your project.
```bash
npx -y react-scan@latest init
```
### Manual Installation
Install the package:
```bash
npm install -D react-scan
```
Then add the script tag to your app. Pick the guide for your framework:
#### Script Tag
Paste this before any scripts in your `index.html`:
```html
```
#### Next.js (App Router)
Add this inside of your `app/layout.tsx`:
```tsx
import Script from "next/script";
export default function RootLayout({ children }) {
return (
{children}
);
}
```
#### Next.js (Pages Router)
Add this into your `pages/_document.tsx`:
```tsx
import { Html, Head, Main, NextScript } from "next/document";
import Script from "next/script";
export default function Document() {
return (
);
}
```
#### Vite
Example `index.html` with React Scan enabled:
```html
```
#### Remix
Add this inside your `app/root.tsx`:
```tsx
import { Links, Meta, Outlet, Scripts } from "@remix-run/react";
export default function App() {
return (
);
}
```
### Browser Extension
Install the extension by following the guide [here](https://github.com/aidenybai/react-scan/blob/main/BROWSER_EXTENSION_GUIDE.md).
## API Reference
Options
```tsx
export interface Options {
/**
* Enable/disable scanning
* @default true
*/
enabled?: boolean;
/**
* Force React Scan to run in production (not recommended)
* @default false
*/
dangerouslyForceRunInProduction?: boolean;
/**
* Log renders to the console
* @default false
*/
log?: boolean;
/**
* Show toolbar bar
* @default true
*/
showToolbar?: boolean;
/**
* Animation speed
* @default "fast"
*/
animationSpeed?: "slow" | "fast" | "off";
onCommitStart?: () => void;
onRender?: (fiber: Fiber, renders: Array) => void;
onCommitFinish?: () => void;
}
```
- `scan(options: Options)`: Imperative API to start scanning
- `useScan(options: Options)`: Hook API to start scanning
- `setOptions(options: Options): void`: Set options at runtime
- `getOptions()`: Get the current options
- `onRender(Component, onRender: (fiber: Fiber, render: Render) => void)`: Hook into a specific component's renders
## Why React Scan?
React can be tricky to optimize.
The issue is that component props are compared by reference, not value. This is intentional -- rendering can be cheap to run.
However, this makes it easy to accidentally cause unnecessary renders, making the app slow. Even production apps with hundreds of engineers can't fully optimize their apps (see [GitHub](https://github.com/aidenybai/react-scan/blob/main/.github/assets/github.mp4), [Twitter](https://github.com/aidenybai/react-scan/blob/main/.github/assets/twitter.mp4), and [Instagram](https://github.com/aidenybai/react-scan/blob/main/.github/assets/instagram.mp4)).
```jsx
alert("hi")} style={{ color: "purple" }} />
```
React Scan helps you identify these issues by automatically detecting and highlighting renders that cause performance issues.
## Resources & Contributing
Want to try it out? Check the [demo](https://react-scan.million.dev).
Looking to contribute? Check the [Contributing Guide](https://github.com/aidenybai/react-scan/blob/main/CONTRIBUTING.md).
Want to talk to the community? Join our [Discord](https://discord.gg/X9yFbcV2rF).
Find a bug? Head to our [issue tracker](https://github.com/aidenybai/react-scan/issues).
[**→ Start contributing on GitHub**](https://github.com/aidenybai/react-scan/blob/main/CONTRIBUTING.md)
## Acknowledgments
- [React Devtools](https://react.dev/learn/react-developer-tools) for the initial idea of highlighting renders
- [Million Lint](https://million.dev) for scanning and linting approaches
- [Why Did You Render?](https://github.com/welldone-software/why-did-you-render) for the concept of detecting unnecessary renders
## License
React Scan is [MIT-licensed](LICENSE) open-source software by Aiden Bai, [Million Software, Inc.](https://million.dev), and [contributors](https://github.com/aidenybai/react-scan/graphs/contributors).