Validating NestJS env vars with zod

Hey there! If you're like me, you probably love using Zod for validating and parsing data objects. It's a lifesaver, right? But when it comes to environment variables (env vars), things can get a bit tricky. You want to parse them as quickly as possible during your application's boot-up phase and then use them confidently throughout your app. That's where NestJS's ConfigModule comes into play.

Setting Up ConfigModule

NestJS provides a handy ConfigModule that lets you register a global module to read your env file and load it into the process. But here's the catch: you need to validate those env vars to ensure they're correct. Luckily, the ConfigModule allows you to register a validate function. Here's a quick look at how you can set it up:

import { ConfigModule } from '@nestjs/config';
 
@Module({
  imports: [
    ConfigModule.forRoot({ isGlobal: true, validate: /* Your function */ }),
  ]
})
export class AppModule {}

Validating with Zod

The validate function receives the full process.env object and expects a validated, parsed object in return. This is where Zod comes in handy. You can use it to format and validate your env vars like this:

import { z } from 'zod';
 
const envs = z.object({
  DATABASE_URL: z.string(),
  NODE_ENV: z.string().optional(),
});
 
export const validate = (config: Record<string, unknown>) => {
  const validated = envs.parse(config);
  return validated;
};

Extending ConfigService Types

As a bonus, you can extend the ConfigService types with your Zod schema. This ensures that whenever you're using the ConfigModule, you have type safety. Here's how you can define your type:

import type { ConfigService } from '@nestjs/config';
 
export type IConfigService = ConfigService<z.infer<typeof envs>>;

Using the Extended ConfigService

Once you've set up your types, you can use them in your modules like so:

@Module({
  imports: [
    DrizzlePostgresModule.registerAsync({
      tag: DB_TAG,
      imports: [ConfigModule],
      inject: [ConfigService],
      useFactory: (configService: IConfigService) => ({
        postgres: {
          url: configService.getOrThrow('DATABASE_URL'),
        }
      }),
    }),
  ],
})
export class DbModule {}

tl;dr

Use NestJS's ConfigModule to load and validate env vars.
Register a validate function to ensure env vars are correct.
Use Zod to format and validate env vars.
Extend ConfigService types with Zod schema for type safety.
Implement the extended types in your modules for better type checking.

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