# InitializeOnPreviewAllowlist

**Kind:** Class

**Source:** [`packages/core/inspector/initialize-on-preview.allowlist.ts`](https://github.com/nestjs/nest/blob/master/packages/core/inspector/initialize-on-preview.allowlist.ts#L3)

**Part of:** [Core](subsystem-packages-core)

`InitializeOnPreviewAllowlist` tracks which component or entity types are permitted to run initialization logic while the inspector is in preview mode. It provides a small allowlist API: use `add()` to register allowed entries and `has()` to check whether preview initialization should be enabled for a given entry.

## Methods

| Method | Signature | Returns |
|---|---|---|
| `add` | `add(type: Type)` | `void` |
| `has` | `has(type: Type)` | `void` |

## Diagram

```mermaid
graph LR
  A[Inspector Preview Flow] --> B[InitializeOnPreviewAllowlist]
  B -->|add(type)| C[Allowed initialization entries]
  A -->|has(type)| B
  B -->|true| D[Run initialization in preview]
  B -->|false| E[Skip initialization]
```

## Usage

```ts
import { InitializeOnPreviewAllowlist } from './initialize-on-preview.allowlist';

const previewAllowlist = new InitializeOnPreviewAllowlist();

// Allow a component type to initialize during inspector preview.
previewAllowlist.add('my-component');

function shouldInitializeInPreview(componentType: string): boolean {
  return previewAllowlist.has(componentType);
}

if (shouldInitializeInPreview('my-component')) {
  // Run preview-safe initialization.
}
```

## AI Coding Instructions

- Use `add()` during inspector or component registration to explicitly opt entries into preview initialization.
- Always guard preview-only initialization with `has()` rather than assuming normal runtime initialization is safe in preview.
- Keep allowlisted initialization side-effect-safe; preview code should avoid unexpected persistence, network calls, or global mutations.
- Reuse the same allowlist instance across the relevant inspector preview flow so registrations are visible to checks.

## Relationships

- IMPORTS → `Type`