Node.js: The Runtime, process, and Configuration

Node.js lets you run JavaScript outside the browser: web servers, command-line tools, automation scripts. It combines:

• V8, Chrome’s JavaScript engine, which runs your code;
• libuv, a library that handles I/O (files, network, timers) asynchronously;
• a standard library of modules (node:fs, node:http, node:crypto…).

Your code runs on a single thread. Slow operations (reading a file, waiting for a request) are handed off to the system; Node keeps processing other tasks and resumes your code when the result arrives. This is the event loop, which we will cover in detail later. An important consequence: a long synchronous computation blocks the entire server.

Compared to the browser: no window, no document, no DOM. On the other hand, globalThis, console, setTimeout, fetch, URL, and structuredClone are available. In production, use an LTS version (even-numbered): Node 26 is the current LTS.

Running code

node --version             # v26.x (current LTS)
node                       # REPL: interactive console (.exit to quit)
node script.js             # run a file
node --watch server.js     # restart automatically on every change
node --env-file=.env app.js  # load a .env file (stable option)
node outil.ts              # run TypeScript: types are stripped, not checked

The process object

process is a global object that represents the running program.

// node saluer.js Ada --majuscules
console.log(process.argv);
// [ '/usr/bin/node', '/home/ada/saluer.js', 'Ada', '--majuscules' ]

console.log(process.platform, process.version, process.cwd());

The first two elements of process.argv are the path to Node and the path to the script. To parse options, util.parseArgs (stable since Node 20) saves you from coding everything by hand:

const { parseArgs } = require("node:util");

const { values, positionals } = parseArgs({
  args: process.argv.slice(2),
  options: {
    port: { type: "string", short: "p", default: "3000" },
    verbose: { type: "boolean", short: "v", default: false },
  },
  allowPositionals: true,
});
// node serve.js -p 8080 -v public  =>  values.port === "8080", positionals = ["public"]

Environment variables

Configuration (port, database URL, API keys) is read from process.env. It is not in the code: this way the same program can be used in development and in production.

const config = {
  port: Number(process.env.PORT ?? 3000),      // values are ALWAYS strings
  nodeEnv: process.env.NODE_ENV ?? "development",
  databaseUrl: process.env.DATABASE_URL,
};
if (!config.databaseUrl) {
  console.error("DATABASE_URL is missing");
  process.exit(1);                             // fail early and clearly
}

Validate the configuration at startup: it is better to refuse to start than to fail in the middle of the night on the first request.

Exit codes

A program that ends returns an exit code: 0 for success, anything else for an error. Shell scripts, CI, and Docker rely on it. Prefer process.exitCode = 1 (the program cleanly finishes its pending tasks) to process.exit(1) (immediate stop), except at startup.

Key takeaways

Node = V8 + libuv + standard library, on a non-blocking thread. process.argv and util.parseArgs for arguments, process.env (strings) for configuration, and an exit code to communicate the result.