# Using ConfigCat's OpenFeature Provider in NestJS

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## Getting started[​](#getting-started "Direct link to Getting started")

OpenFeature offers a [NestJS SDK](https://github.com/open-feature/js-sdk/tree/main/packages/nest) to streamline the use of OpenFeature in NestJS applications. This SDK is built on top of the [OpenFeature JavaScript Node.js SDK](https://github.com/open-feature/js-sdk/blob/main/packages/server).

Since ConfigCat implements [a provider](https://configcat.com/docs/sdk-reference/openfeature/node.md) for the Node.js SDK, you can use ConfigCat with the OpenFeature NestJS SDK, as explained below.

### 1. Install the NestJS SDK and the ConfigCat provider[​](#1-install-the-nestjs-sdk-and-the-configcat-provider "Direct link to 1. Install the NestJS SDK and the ConfigCat provider")

```bash
npm i @openfeature/nestjs-sdk @openfeature/config-cat-provider

```

### 2. Initialize the NestJS SDK[​](#2-initialize-the-nestjs-sdk "Direct link to 2. Initialize the NestJS SDK")

The `ConfigCatProvider.create()` function takes the SDK key, the desired [polling mode](https://configcat.com/docs/sdk-reference/js/node.md#polling-modes) (Auto Polling is recommended for this kind of application) and an optional `options` argument containing additional configuration options for the underlying [ConfigCat client](https://configcat.com/docs/sdk-reference/js/node.md#creating-the-configcat-client):

```ts
import { Module } from '@nestjs/common';
import { OpenFeatureModule } from '@openfeature/nestjs-sdk';
import { ConfigCatProvider } from '@openfeature/config-cat-provider';
import { createConsoleLogger, LogLevel, PollingMode } from '@configcat/sdk';
import { FeatureFlagService } from './_services/feature-flag.service';

const configCatProvider = ConfigCatProvider.create('#YOUR-SDK-KEY#', PollingMode.AutoPoll, {
  // Specify options for the underlying ConfigCat client
  logger: createConsoleLogger(LogLevel.Info),
  setupHooks: (hooks) => hooks.on('clientReady', () => console.log('Client is ready!')),
  // ...
});

@Module({
  imports: [
    OpenFeatureModule.forRoot({
      defaultProvider: configCatProvider,
      // Set the context for your evaluations
      contextFactory: (request) => ({ targetingKey: 'user-1' })
    }),
  ],
  controllers: [/* ... */],
  providers: [FeatureFlagService],
})
export class AppModule {}

```

### 3. Use your feature flag[​](#3-use-your-feature-flag "Direct link to 3. Use your feature flag")

To improve maintainability and testability, it's usually a good idea not to directly evaluate your feature flags, but to wrap them in a service:

```ts
import { Injectable } from '@nestjs/common';
import { OpenFeatureClient, Client } from '@openfeature/nestjs-sdk';

@Injectable()
export class FeatureFlagService {
  constructor(
    @OpenFeatureClient() private readonly defaultClient: Client,
  ) {
  }

  public async isAwesomeFeatureEnabled() {
    return await this.defaultClient.getBooleanValue('isAwesomeFeatureEnabled', false);
  }
}

```

Then you can inject this service into your controllers and use it to evaluate feature flags:

```ts
import { Controller, Get } from '@nestjs/common';
import { FeatureFlagService } from './_services/feature-flag.service';

@Controller()
export class AppController {
  constructor(
    private readonly featureFlagService: FeatureFlagService
  ) {}

  @Get()
  async welcome() {
    return await this.featureFlagService.isAwesomeFeatureEnabled()
      ? 'Awesome feature is enabled!'
      : 'Awesome feature is disabled.';
  }
}

```

## Evaluation Context[​](#evaluation-context "Direct link to Evaluation Context")

An [evaluation context](https://openfeature.dev/docs/reference/concepts/evaluation-context) in the OpenFeature specification is a container for arbitrary contextual data that can be used as a basis for feature flag evaluation.

You can find [here](https://configcat.com/docs/sdk-reference/openfeature/node.md#evaluation-context) how the ConfigCat provider translates these evaluation contexts to ConfigCat [User Objects](https://configcat.com/docs/sdk-reference/js/node.md#user-object).

## Advanced features[​](#advanced-features "Direct link to Advanced features")

Read the documentation of the [OpenFeature NestJS SDK](https://openfeature.dev/docs/reference/technologies/server/javascript/nestjs/) to learn more about the advanced capabilities of the SDK.

## Look under the hood[​](#look-under-the-hood "Direct link to Look under the hood")

* [ConfigCat OpenFeature Provider's repository on GitHub](https://github.com/open-feature/js-sdk-contrib/tree/main/libs/providers/config-cat)
* [ConfigCat OpenFeature Provider in NPM](https://www.npmjs.com/package/@openfeature/config-cat-provider)