How do I load dotenv in Typescript

Question

I'm trying to load .env environment variable using Typescript.

This is my .env and app.ts files

//.env

DB_URL=mongodb://127.0.0.1:27017/test

// app.ts

import * as dotenv from 'dotenv';
import express from 'express';
import mongoose from 'mongoose';

dotenv.config();
mongoose.connect(process.env.DB_URL, config);

When I run app.ts using ts-node src/app.ts command, throwing this error

Unable to compile TypeScript:
src/app.ts:50:18 - error TS2769: No overload matches this call.
  Overload 1 of 3, '(uris: string, callback: (err: MongoError) => void): Promise<typeof import("mongoose")>', gave the following error.
    Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
      Type 'undefined' is not assignable to type 'string'.
  Overload 2 of 3, '(uris: string, options?: ConnectionOptions | undefined): Promise<typeof import("mongoose")>', gave the following error.
    Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
      Type 'undefined' is not assignable to type 'string'.

50 mongoose.connect(process.env.DB_URL, config);

But when I add below if statement, it works well

//app.ts
import * as dotenv from 'dotenv';
import express from 'express';
import mongoose from 'mongoose';

dotenv.config();

//Add this code
if (!process.env.DB_URL) {
  process.exit(1);
}

mongoose.connect(process.env.DB_URL, config);

Example app listening at http://localhost:3000
Mongoose default connection is open to  mongodb://127.0.0.1:27017/test

I want to know why is this error not thrown in below code?

Answer 1

From the error message, you can conclude that TypeScript expect the first parameter of mongoose.connect() to be a string but environment variables can be either string or undefined.

By adding the condition, you eliminate the possibility that process.env.DB_URL would be undefined when at the time you call mongoose.connect(process.env.DB_URL, config)

If you are certain that it will never be undefined, you can use TypeScript Non-null assertion operator

mongoose.connect(process.env.DB_URL!, config);

Tip:

Instead of

import * as dotenv from 'dotenv';
...
dotenv.config();

You can do as the following

import 'dotenv/config'

This will make sure environment variables are set before the next import statements, so you would only have to do this only once at the main file of your app.