A flexible feature flag module for NestJS applications. Easily manage feature toggles using decorators and configuration.

Description

nestjs-flags provides a simple way to implement feature flags (also known as feature toggles) in your NestJS application. It integrates with the standard @nestjs/config module to load flag statuses and offers a Guard and Decorator (@FeatureFlag) to control access to routes based on whether a feature is enabled or disabled.

Features

• Loads flags from configuration (via @nestjs/config).
• Provides NestjsFlagsService for imperative flag checks.
• Provides @FeatureFlag() decorator and FeatureFlagGuard for declarative route protection.
• Configurable via forRoot options (custom config key, custom exception on disabled flag).
• Written in TypeScript with types included.
• Unit tested.

Installation

npm install nestjs-flags
# or
yarn add nestjs-flags

Quick Start

1. Configure your flags: Set up your flag configuration source, for example, using a .env file:

   # Feature flags (prefix matches default 'featureFlags' key mapping)
   FEATURE_FLAGS_NEW_USER_PROFILE=true
   FEATURE_FLAGS_EXPERIMENTAL_SEARCH=false

2. Import ConfigModule and NestjsFlagsModule: In your main application module (e.g., app.module.ts), import and configure @nestjs/config and NestjsFlagsModule.

   // src/app.module.ts
   import { Module, ForbiddenException } from "@nestjs/common";
   import { ConfigModule } from "@nestjs/config";
   import { AppController } from "./app.controller";
   import { AppService } from "./app.service";
   import { NestjsFlagsModule } from "nestjs-flags"; // Import the module
   
   @Module({
     imports: [
       ConfigModule.forRoot({
         isGlobal: true,
         envFilePath: ".env",
         load: [finalConfig],
       }),
   
       NestjsFlagsModule.autoLoadFromEnv(),
     ],
     controllers: [AppController],
     providers: [AppService],
   })
   export class AppModule {}

3. Use the Service or Decorator:

   • Using NestjsFlagsService: Inject the service to check flags programmatically.

     // src/app.controller.ts
     import { Controller, Get, Logger } from "@nestjs/common";
     import { NestjsFlagsService } from "nestjs-flags";
     
     @Controller("service-example")
     export class ServiceExampleController {
       constructor(private readonly flagsService: NestjsFlagsService) {}
     
       @Get("feature")
       getFeature() {
         const isNewUserProfileEnabled =
           this.flagsService.isFeatureEnabled("newUserProfile");
     
         if (isNewUserProfileEnabled) {
           return { message: "New user profile is available!" };
         } else {
           return { message: "Showing the old profile." };
         }
       }
     }

   • Using @FeatureFlag Decorator and Guard: Protect entire routes declaratively.

     // src/app.controller.ts
     import { Controller, Get, UseGuards, Logger } from "@nestjs/common";
     import { FeatureFlag, FeatureFlagGuard } from "nestjs-flags";
     
     @Controller("decorator-example")
     export class DecoratorExampleController {
       private readonly logger = new Logger(DecoratorExampleController.name);
     
       @Get("profile")
       @UseGuards(FeatureFlagGuard) // Activate the guard
       @FeatureFlag("newUserProfile") // Specify the flag to check
       getProfileFeature() {
         // This code only runs if 'newUserProfile' is true
         this.logger.log("Guard allowed access to profile.");
         return { message: "Showing the NEW user profile!" };
       }
     
       @Get("search")
       @UseGuards(FeatureFlagGuard)
       @FeatureFlag("experimentalSearch")
       getSearchFeature() {
         // This code only runs if 'experimentalSearch' is true
         // If false, the guard throws an exception (NotFoundException by default)
         this.logger.log("Guard allowed access to search.");
         return { message: "Using the EXPERIMENTAL search!" };
       }
     }