Environment and Configuration

Accessing environment variables

JavaScript doesn't provide access to environment variables natively. However, some non-browser JavaScript runtimes, such as Node.js and Node provides access to environment variables.

In Node.js:

const name = "EXAMPLE_VARIABLE";

let value = process.env[name];
if (!value) {
    console.log(`Variable '${name}' not set.`);
} else {
    console.log(`Variable '${name}' set to '${value}'.`);
}

In Deno:

const name = "EXAMPLE_VARIABLE";

let value = Deno.env.get(name);
if (!value) {
    console.log(`Variable '${name}' not set.`);
} else {
    console.log(`Variable '${name}' set to '${value}'.`);
}

Rust is providing the same functionality of accessing an environment variable at runtime via the var and var_os functions from the std::env module.

var function is returning a Result<String, VarError>, either returning the variable if set or returning an error if the variable is not set or it is not valid Unicode.

var_os has a different signature giving back an Option<OsString>, either returning some value if the variable is set, or returning None if the variable is not set. An OsString is not required to be valid Unicode.

use std::env;


fn main() {
    let key = "ExampleVariable";
    match env::var(key) {
        Ok(val) => println!("{key}: {val:?}"),
        Err(e) => println!("couldn't interpret {key}: {e}"),
    }
}

use std::env;

fn main() {
    let key = "ExampleVariable";
    match env::var_os(key) {
        Some(val) => println!("{key}: {val:?}"),
        None => println!("{key} not defined in the enviroment"),
    }
}

Rust is also providing the functionality of accessing an environment variable at compile time. The env! macro from std::env expands the value of the variable at compile time, returning a &'static str. If the variable is not set, an error is emitted.

use std::env;

fn main() {
    let example = env!("ExampleVariable");
    println!("{example}");
}

Configuration

JavaScript doesn't support configurations.

In Rust it is available via use of third-party crates such as figment or config. See the following example making use of config crate:

use config::{Config, Environment};

fn main() {
    let builder = Config::builder().add_source(Environment::default());

    match builder.build() {
        Ok(config) => {
            match config.get_string("examplevar") {
                Ok(v) => println!("{v}"),
                Err(e) => println!("{e}")
            }
        },
        Err(_) => {
            // something went wrong
        }
    }
}