Environment variables

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.

:::note

When multiple declarations for the same environment variable exist within a single .env file, the first occurrence is applied. However, if the same variable is defined across multiple .env files (using multiple --env-file arguments), the value from the last file specified takes precedence. This means that the first occurrence found in the last .env file listed will be applied.

:::

@std/dotenv

The dotenv package in the standard library can be used to load environment variables from .env.

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 { load } from "jsr:@std/dotenv";

const env = await load({
  // optional: choose a specific path (defaults to ".env")
  envPath: ".env.local",
  // optional: also export to the process environment (so Deno.env can read it)
  export: true,
});

console.log(env.GREETING);
console.log(Deno.env.get("GREETING"));

Run this with deno run --allow-read --allow-env app.ts.

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"
    }
  }
}

:::note Variables with spaces

When setting environment variables that contain space characters in a .env file, ensure you enclose the value in quotes. For example:

MY_VAR="my value with spaces"

:::

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.