# Write a plugin

Let's inject a global component as an example to see how to define and use plugins.

### 1. Define a plugin

```tsx title="plugin.ts"
import { DocPlugin } from '@modern-js/doc-tools';

export function pluginMonitor(slug: string): DocPlugin {
  // Component path, you need to implement the content of the component yourself
  const componentPath = path.join(__dirname, 'Example.tsx');
  return {
    name: 'plugin-monitor',
    // Path to global components
    globalUIComponents: [componentPath],
    // Global variable definitions for build phase
    builderConfig: {
      source: {
        define: {
          'process.env.SLUG': JSON.stringify(slug),
        },
      },
    },
  };
}
```

```tsx title="Example.tsx"
import React from 'react';

const Example = () => {
  console.log(process.env.SLUG);
  return <div>Example</div>;
};

export default Example;
```

A plugin is generally a function that receives some plugin params (optional) and returns an object that contains the name of the plugin and other config.

In the above example, we define a plugin named `plugin-example`, which will define a global environment variable `slug` during the build phase, and inject a global component `Example.tsx` in the document.

### 2. Use a plugin

Register plugins via `doc.plugins` in `modern.config.ts`:

```tsx title="modern.config.ts"
import { pluginExample } from './plugin';

export default {
  doc: {
    plugins: [pluginMonitor('test')],
  },
};
```

Then the `Example` component will be injected into the page and we can access the `slug` variable in the component.