# createParamDecorator

**Kind:** Function

**Source:** [`packages/common/decorators/http/create-route-param-metadata.decorator.ts`](https://github.com/nestjs/nest/blob/master/packages/common/decorators/http/create-route-param-metadata.decorator.ts#L19)

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

Defines HTTP route param decorator

`createParamDecorator` creates custom parameter decorators for HTTP route handlers. It stores parameter metadata and invokes a factory callback at request time, allowing handlers to receive values derived from the request, response, or execution context.

## Signature

```ts
function createParamDecorator(factory: CustomParamFactory<FactoryData, FactoryOutput>, enhancers: ParamDecoratorEnhancer[]): ( ...dataOrPipes: (Type<PipeTransform> | PipeTransform | FactoryData)[] ) => ParameterDecorator
```

## Parameters

| Name | Type |
|---|---|
| `factory` | `CustomParamFactory<FactoryData, FactoryOutput>` |
| `enhancers` | `ParamDecoratorEnhancer[]` |

**Returns:** `( ...dataOrPipes: (Type<PipeTransform> | PipeTransform | FactoryData)[] ) => ParameterDecorator`

## Diagram

```mermaid
graph LR
  A[HTTP Request] --> B[Route Handler]
  B --> C[Custom Parameter Decorator]
  C --> D[Factory Callback]
  D --> E[ExecutionContext]
  E --> F[Extracted Parameter Value]
  F --> G[Handler Method Argument]
```

## Usage

```ts
import { createParamDecorator, ExecutionContext } from '@nestjs/common';

export const CurrentUser = createParamDecorator(
  (property: string | undefined, ctx: ExecutionContext) => {
    const request = ctx.switchToHttp().getRequest();
    const user = request.user;

    return property ? user?.[property] : user;
  },
);

// Usage in a controller
@Get('profile')
getProfile(
  @CurrentUser() user: User,
  @CurrentUser('id') userId: string,
) {
  return { user, userId };
}
```

## AI Coding Instructions

- Use the factory callback to extract values from `ExecutionContext`, typically through `ctx.switchToHttp().getRequest()`.
- Treat the optional `data` argument as decorator configuration, such as a property name or lookup key.
- Keep decorators focused on extracting request-scoped values; place business logic in guards, services, or controllers.
- Ensure extracted values handle missing request properties safely, especially for optional authentication data.
- Use the resulting decorator on controller method parameters and combine it with pipes when validation or transformation is needed.