Introduction:

In this article, we will explore how to validate environment variables in a NestJS application. Validating environment variables is crucial for ensuring that your application is running with the correct configuration and helps prevent runtime errors due to misconfigured or missing variables. We will use the @nestjs/config, class-transformer, and class-validator packages to achieve this.

Step 1: Install Dependencies
First, let's install the required dependencies:

npm i --save @nestjs/config class-transformer class-validator

Step 2: Create a Validation File
Create a new file named src/env.validation.ts. In this file, we will define the environment variables we want to validate, such as database configuration variables.

Here's an example of the env.validation.ts file:

import { plainToInstance } from 'class-transformer';
import { IsNumber, IsString, validateSync } from 'class-validator';

class EnvironmentVariables {
    @IsString()
    DATABASE_HOST: string;

    @IsNumber()
    DATABASE_PORT: number;

    @IsString()
    DATABASE_USERNAME: string;

    @IsString()
    DATABASE_PASSWORD: string;

    @IsString()
    DATABASE_NAME: string;
}

export function validate(config: Record<string, unknown>) {
    const validatedConfig = plainToInstance(
        EnvironmentVariables,
        config,
        { enableImplicitConversion: true },
    );
    const errors = validateSync(validatedConfig, { skipMissingProperties: false });

    if (errors.length > 0) {
        throw new Error(errors.toString());
    }
    return validatedConfig;
}

In this file, we define a class EnvironmentVariables with the required properties and their validation decorators. The validate function takes a configuration object, converts it to an instance of EnvironmentVariables, and validates it using validateSync. If there are any validation errors, an exception is thrown.

Step 3: Load ConfigModule in AppModule
Next, we need to load the ConfigModule in our app.module.ts file and provide the validate function we created earlier:

import { Module } from '@nestjs/common';
import { ConfigModule } from '@nestjs/config';
import { AppController } from './app.controller';
import { AppService } from './app.service';
import { validate } from './env.validation';

@Module({
  imports: [
    ConfigModule.forRoot({
      validate,
      isGlobal: true
    }),
  ],
  controllers: [AppController],
  providers: [AppService],
})
export class AppModule { }

Step 4: Create a .env File
Create a .env file in the root of your project with the required environment variables. For example:

DATABASE_HOST=postgres
DATABASE_PORT=5432
DATABASE_USERNAME=postgres
DATABASE_PASSWORD=dev-secret
DATABASE_NAME=postgres

Now, when you run your NestJS application, it should start successfully if all environment variables are correctly configured.

However, if any environment variable is missing or has an incorrect value, an exception will be thrown, and the application will not start.

For example, if we comment out the DATABASE_NAME variable in the .env file and restart the application:

Conclusion:

Validating environment variables in your NestJS application is an effective way to ensure that your application is running with the correct configuration and to catch errors early in the development process. By using the @nestjs/config, class-transformer, and class-validator packages, you can easily implement robust validation for your environment variables.