There are a few ways to use environment variables in Deno:

Built-in Deno.env method

The Deno runtime offers built-in support for environment variables with Deno.env.

Deno.env has getter and setter methods. Here is example usage:

Deno.env.set("FIREBASE_API_KEY", "examplekey123");
Deno.env.set("FIREBASE_AUTH_DOMAIN", "firebasedomain.com");

console.log(Deno.env.get("FIREBASE_API_KEY")); // examplekey123
console.log(Deno.env.get("FIREBASE_AUTH_DOMAIN")); // firebasedomain.com
console.log(Deno.env.has("FIREBASE_AUTH_DOMAIN")); // true

.env file

Deno also supports .env files. You can tell Deno to read environment variables from .env with the --env-file flag, for example:

deno run --env-file main.ts

This will read the .env file from the current working directory or the first parent directory that contains one. If you want to load environment variables from a different file, you can specify that file as a parameter to the flag.

You can pass multiple --env-file flags (e.g., deno run --env-file=.env.one --env-file=.env.two --allow-env <script>) to load variables from multiple files.

Alternately, the dotenv package in the standard library will load environment variables from .env as well.

Let's say you have an .env file that looks like this:

GREETING="Hello, world."

Import the load module to auto-import from the .env file and into the process environment.

import "jsr:@std/dotenv/load";

console.log(Deno.env.get("GREETING")); // "Hello, world."

Further documentation for .env handling can be found in the @std/dotenv documentation.

Set a variable when running a command

As with other CLI commands, you can set environment variables before running a command like so:

MY_VAR="my value" deno run main.ts

This can be useful when you want to vary a task based on an environment variable, and can be helpfully combined with deno task commands like so:

{

  ...
  
  "tasks": {
    "build:full": {
      "description": "Build the site with all features",
      "command": "BUILD_TYPE=FULL deno run main.ts"
    },
    "build:light": {
      "description": "Build the site without expensive operations",
      "command": "BUILD_TYPE=LIGHT deno run main.ts"
    }
  }
}

std/cli

The Deno Standard Library has a std/cli module for parsing command line arguments. Please refer to the module for documentation and examples.

Special environment variables

The Deno runtime has these special environment variables.