> For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt.

# Introduce

Modern.js runtime configuration should be centralized in the `src/modern.runtime.ts` file.

:::info

If this file doesn't exist in your project yet, create it with the following command:

```bash
touch src/modern.runtime.ts
```

:::

## Basic Configuration

```tsx
import { defineRuntimeConfig } from '@modern-js/runtime';

export default defineRuntimeConfig({
  router: {
    // Router configuration
  },
  // Other runtime modules...
});
```

## Multi-Entry Configuration

For multi-entry applications, `defineRuntimeConfig` can accept a function that returns specific configurations based on the entry name:

```tsx
import { defineRuntimeConfig } from '@modern-js/runtime';

export default defineRuntimeConfig(entryName => {
  if (entryName === 'main') {
    return {
      router: {
        // Router configuration for "main" entry
      },
    };
  }

  // Configuration for other entries
  return {
    router: {
      // Other entry router configuration
    },
  };
});
```

:::tip

Using the `src/modern.runtime.ts` configuration approach does not support exporting asynchronous functions, which is related to the rendering method of Modern.js. If you need to add asynchronous logic, please use the **[Runtime Plugin](/plugin/introduction.md#runtime-plugin)**.

:::