Environment variables in NestJS - Step-by-Step Execution

Concept Flow - Environment variables

Start Application

↓

Load .env file

↓

Parse variables

↓

Set process.env variables

↓

Access variables in code

↓

Use variables for config

↓

Application runs with config

The app starts by loading a .env file, parsing its variables, setting them in process.env, then accessing them in code for configuration.

Execution Sample

NestJS

import { Module } from '@nestjs/common';
import { ConfigModule } from '@nestjs/config';

@Module({
  imports: [ConfigModule.forRoot()],
})
export class AppModule {}

This code loads environment variables from a .env file into process.env when the app starts.

Execution Table

Step	Action	process.env state	Result
1	Application starts	{}	No env vars loaded yet
2	ConfigModule.forRoot() loads .env	{"PORT":"3000","DB_HOST":"localhost"}	Env vars parsed and set
3	Access process.env.PORT	{"PORT":"3000","DB_HOST":"localhost"}	Returns '3000'
4	Access process.env.DB_HOST	{"PORT":"3000","DB_HOST":"localhost"}	Returns 'localhost'
5	Use env vars in config	{"PORT":"3000","DB_HOST":"localhost"}	App configures server and DB connection
6	Application runs	{"PORT":"3000","DB_HOST":"localhost"}	App uses env vars for behavior

💡 All environment variables loaded and used; app runs with config from .env

Variable Tracker

Variable	Start	After Step 2	After Step 6
process.env	{}	{"PORT":"3000","DB_HOST":"localhost"}	{"PORT":"3000","DB_HOST":"localhost"}

Key Moments - 3 Insights

Why do we not see environment variables before ConfigModule.forRoot() runs?

Because process.env is empty initially (see Step 1 in execution_table). The ConfigModule loads and sets variables at Step 2.

How do we access environment variables in NestJS code?

We use process.env.VARIABLE_NAME after ConfigModule.forRoot() has loaded them, as shown in Steps 3 and 4.

What happens if a variable is missing in the .env file?

It won't be set in process.env, so accessing it returns undefined. You should provide defaults or validation.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of process.env after Step 2?

A {"PORT":"3000","DB_HOST":"localhost"}

B {}

C {"PORT":"undefined","DB_HOST":"localhost"}

D null

💡 Hint

Check the 'process.env state' column at Step 2 in the execution_table.

At which step does the application start using environment variables for configuration?

A Step 1

B Step 3

C Step 5

D Step 6

💡 Hint

Look for when the app uses env vars for config in the 'Result' column.

If the .env file did not have DB_HOST, what would process.env.DB_HOST be after Step 2?

A "localhost"

B undefined

C "" (empty string)

D null

💡 Hint

Variables not in .env are not set in process.env, see key_moments about missing variables.

Concept Snapshot

NestJS loads environment variables from a .env file using ConfigModule.forRoot().
These variables become available in process.env.
Access them in your code via process.env.VARIABLE_NAME.
Missing variables return undefined.
Use env vars to configure app behavior without hardcoding.

Full Transcript

When a NestJS application starts, it uses ConfigModule.forRoot() to load environment variables from a .env file. This process reads the file, parses key-value pairs, and sets them in the Node.js process.env object. Before loading, process.env is empty for these variables. After loading, variables like PORT and DB_HOST are accessible anywhere in the app via process.env.PORT or process.env.DB_HOST. These values configure the app's behavior, such as which port to listen on or database connection details. If a variable is missing in the .env file, accessing it returns undefined, so defaults or validation are recommended. This approach keeps configuration separate from code, making apps flexible and secure.