Actually you have define the path of the .env file

Try like this

import * as dotenv from "dotenv";
dotenv.config({ path: __dirname+'/.env' });

Try this also

require('dotenv').config({ path: __dirname+'/.env' });

Change the path of .env file as required

reference : https://www.npmjs.com/package/dotenv

You could use built-in NestJs way to handle this (ConfigModule).

Configuration | NestJs

// main.ts

import { Module } from '@nestjs/common';
import { ConfigModule } from '@nestjs/config';
import { InventoriesModule } from './inventories/inventories.module';
import typeORMConfig from './config/database.config';
import { TypeOrmModule } from '@nestjs/typeorm';
import { PrismaModule } from './prisma/prisma.module';

@Module({
  imports: [
    ConfigModule.forRoot(), // load .env file
    TypeOrmModule.forRoot(typeORMConfig()),
    InventoriesModule,
    PrismaModule
  ],
})
export class AppModule { }

// ./config/database.config

import { TypeOrmModuleOptions } from '@nestjs/typeorm';

export default function (): TypeOrmModuleOptions {
    return {
        'type': 'mysql',
        'host': process.env.DB_HOST,
        'port': 3306,
        'username': process.env.DB_USERNAME,
        'password': process.env.DB_PASSWORD,
        'database': process.env.DB_DATABASE,
        'entities': ['dist/**/*.entity{.ts,.js}'],
        'synchronize': false
    }
};

If this is a React app and you are using react-script you need to prefix the key with REACT_APP_ otherwise they will be ignored.

REACT_APP_TYPE=xxx
REACT_APP_HOST=xxx
REACT_APP_PORT=xxx
REACT_APP_USERNAME=xxx
REACT_APP_PASSWORD=xxx
REACT_APP_DATABASE=xxx

Ref -> https://create-react-app.dev/docs/adding-custom-environment-variables/

My project has eslint setup, so I have to disable the import/first rule

/* eslint-disable import/first */
require('dotenv').config();

import Koa from 'koa';

import { Logger } from './utils/loggers';
import { app } from './app';

const LOGGER = Logger();
const port = parseInt(process.env.PORT as string, 10) || 8081;

const server = (server: Koa) => {
  server.listen(port, () => {
    LOGGER.info(`> Ready on http://localhost:${port}`);
  });
};

server(app());

We can also use:

import 'dotenv/config'

but

require('dotenv').config({path:path_to_dotenv});

is more flexiable.

If you get undefined value and if you use ES6, you need to import it as follows (.env file must be in project root directory) :

How do I use dotenv with import?

1. Preload dotenv: node --require dotenv/config index.js (Note: you do not need to import dotenv with this approach)
2. Import dotenv/config instead of dotenv (Note: you do not need to call dotenv.config() and must pass options via the command line or environment variables with this approach)
3. Create a separate file that will execute config first as outlined in this comment on #133

You have to import in your project's app.ts file (first) Example with express:

app.ts

//here
import 'dotenv/config'

import express from 'express'
import { userRouter } from './routes/user'

const app = express()

app.use(`/users`, userRouter)
app.listen(process.env.PORT, () => {
    console.log(`running`)
})

Now use it anywhere in your project

It is always good to read the documentation

You must import the dotenv and execute dotenv.config() at the top most of root file.

As early as possible in your application, require and configure dotenv. Ref:dotenv-npm

This is Nestjs way how to set environment variables:

 npm install @nestjs/config

That packages internally uses dotenv package which puts together all environment variables in your machine.

app.module.ts

// configModule chooses the .env file, configservice extract the settings
import { ConfigModule, ConfigService } from '@nestjs/config';


@Module({
  imports: [
    // list your project's modules
    ConfigModule.forRoot({
      // this is set so we do not have to reimport the ConfigModule all over the place into other modules
      isGlobal: true,
      envFilePath: `.env.${process.env.NODE_ENV}`,
    }),
    // Notice we are not using TypeOrmModule.forRoot({})
    // we set this to get access to ConfigService through dependency injection system
    TypeOrmModule.forRootAsync({
      // this tell DI system, find the configService which has all of the config info
      inject: [ConfigService],
      useFactory: (config: ConfigService) => {
        return {
          type: 'sqlite',
          database: config.get<string>('DATABASE'),
          synchronize: true,
          entities: [User, Report],
        };
      },
    }),

I had this issue too, it was driving me nuts until I realized I copied pasted environment values from a YAML file which does not have the same format than the .env file.

One uses :, the other uses =.

If it helps another developer soul :-)