# InvalidDecoratorItemException

**Kind:** Class

**Source:** [`packages/common/utils/validate-each.util.ts`](https://github.com/nestjs/nest/blob/master/packages/common/utils/validate-each.util.ts#L1)

**Part of:** [Common](subsystem-packages-common)

`InvalidDecoratorItemException` represents an invalid item encountered while validating decorator metadata or decorator-provided collections. It provides a `what()` method that returns a human-readable diagnostic message, allowing validation and tooling code to report the failure consistently.

**Extends:** `Error`

## Methods

| Method | Signature | Returns |
|---|---|---|
| `what` | `what()` | `string` |

## Diagram

```mermaid
graph LR
  A[Decorator metadata validation] --> B[Invalid decorator item detected]
  B --> C[InvalidDecoratorItemException]
  C --> D[what(): string]
  D --> E[Readable diagnostic message]
```

## Usage

```ts
import { InvalidDecoratorItemException } from '@nestjs/common';

try {
  // Run code that processes or validates decorator metadata.
  initializeApplicationMetadata();
} catch (error) {
  if (error instanceof InvalidDecoratorItemException) {
    console.error(`Decorator configuration error: ${error.what()}`);
  }

  throw error;
}
```

## AI Coding Instructions

- Use `InvalidDecoratorItemException` for invalid decorator items rather than throwing generic `Error` instances.
- Call `what()` when presenting the error in logs, diagnostics, or developer-facing validation output.
- Preserve the original exception when catching it; avoid replacing it with a less specific error type.
- Keep decorator validation failures actionable by ensuring invalid items can be identified by the surrounding validation logic.