# Discovery

This module provides access to the `DiscoveryService` which can be used to query the various modules, providers, controllers and handlers that make up your NestJS application.

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## Getting Started

::: code-group

```bash [npm]
npm install ---save @golevelup/nestjs-discovery
```

```bash [yarn]
yarn add @golevelup/nestjs-discovery
```

```bash [pnpm]
pnpm add @golevelup/nestjs-discovery
```

:::

## Import

Import and add `DiscoveryModule` to the `imports` section of the module you wish to implement Discovery features in. It's common to inject it directly into consuming Module's contructor so that it can be used during the `onModuleInit` lifecycle hook at application startup.

```typescript
import { DiscoveryModule } from '@golevelup/nestjs-discovery';
import { Module } from '@nestjs/common';

@Module({
  imports: [DiscoveryModule],
})
export class ExampleModule implements OnModuleInit {
  constructor(private readonly discover: DiscoveryService) {}

  public async onModuleInit() {
    // const providers = await this.discover.providersWithMetaAtKey<number>('metaKey')
  }
}
```

## Discover

The `DiscoveryService` exposes several different querying patterns for your app's components that are [well documented with comments](src/discovery.service.ts). This will also provide intellisense for querying in a TypeScript compatible IDE.

In the case of querying for `providers` or `controllers`, the service returns the following interfaces:

```typescript
export interface DiscoveredModule {
  name: string;
  instance: {};
  injectType?: Type<{}>;
  dependencyType: Type<{}>;
}

export interface DiscoveredClass extends DiscoveredModule {
  parentModule: DiscoveredModule;
}
```

This gives access to the (singleton) `instance` of the matching provider or controller created by the NestJS Dependency Injection container.

The `injectType` can contain the constructor function of the provider token if it is provided as an @Injectable class. In the case of custom providers, this value will either contain the type of the factory function that created the dependency, or undefined if a value was directly provided with `useValue`.

The `dependencyType` is a shortcut to retrieve the constructor function of the actual provided dependency itself. For @Injectable providers/controllers this will simply be the decorated class but for dyanmic providers it will return the constructor function of whatever dependency was actually returned from `useValue` or `useFactory`.

It also provides the string based name for convenience. A `DiscoveredClass` contains a `parentModule` which provides the same set of information for the `@Module` class that the dependency was discovered in.

When querying for methods on `providers` or `controllers` the following interface is returned:

```typescript
export interface DiscoveredMethod {
  handler: (...args: any[]) => any;
  methodName: string;
  parentClass: DiscoveredClass;
}
```

This gives access to the `handler` which is the actual class method implementation as well as the ability to navigate back up the dependency tree with the attached `parentClass`.

When specifically querying for components in the context of looking for decorator metadata, the `...WithMetaAtKey<T>` service methods return the types above along with the metadata that was discovered.

```typescript
export interface DiscoveredMethodWithMeta<T> {
  discoveredMethod: DiscoveredMethod;
  meta: T;
}

export interface DiscoveredClassWithMeta<T> {
  discoveredClass: DiscoveredClass;
  meta: T;
}
```

## Example

Assuming you were using a custom decorator in your application that attached metadata at a key called `exampleKey`:

```typescript
const ExampleDecorator = (meta: string) => SetMetadata('exampleKey', meta);
```

Find all controller methods that have been decorated with `@ExampleDecorator` and retrieve the value they set for meta:

```typescript
const exampleMethodsMeta =
  await this.discover.controllerMethodsWithMetaAtKey<string>('exampleKey');
```