Node.js Fundamentals & Runtime Architecture

// This works in Node.js but not in a browser
const fs = require('node:fs');
const data = fs.readFileSync('/etc/hostname', 'utf-8');

// This works in a browser but not in Node.js (no DOM exists)
// document.getElementById('app').innerText = 'Hello';

Common follow-ups: Why can't a typical browser-based JavaScript library that manipulates the DOM be used directly in a Node.js server application?;What specific security sandboxing does a browser apply to JavaScript that Node.js deliberately doesn't, given its different intended use case?

// process.versions shows exactly which V8 version this Node.js runtime is using
console.log(process.versions.v8); // e.g. '11.3.244.8'

// New JavaScript language features become available in Node.js
// as soon as the V8 version it ships supports them

Common follow-ups: Why does upgrading Node.js sometimes unlock new JavaScript language features, given Node itself doesn't directly implement the language spec?;How does V8's JIT compilation strategy (interpreting first, then optimizing hot code paths) affect a script's performance characteristics over its running lifetime?

# Checking and managing Node.js versions, commonly with a version manager like nvm
nvm ls-remote --lts   # lists available LTS versions
nvm install --lts     # installs the latest LTS release
nvm use 20            # switches to a specific major version

Common follow-ups: What's the practical difference between the 'Current', 'Active LTS', and 'Maintenance LTS' phases in terms of what kind of updates each still receives?;How do you plan and execute an upgrade from one major LTS version to the next with minimal production risk?

// This doesn't block the single thread -- Node.js moves on immediately
// to handle other requests while this database query is in flight
app.get('/users/:id', async (req, res) => {
  const user = await db.users.findById(req.params.id); // non-blocking wait
  res.json(user);
});
// Meanwhile, thousands of OTHER concurrent requests can be handled
// on this same single thread while this one awaits its database response

Common follow-ups: Why does this same single-threaded model become a liability specifically for CPU-bound (rather than I/O-bound) workloads?;How does this event-driven model compare conceptually to a traditional multi-threaded server architecture in terms of memory overhead per concurrent connection?

npm init -y              # creates a new package.json
npm install express      # installs a package and adds it as a dependency
npm install --save-dev jest  # installs as a dev-only dependency
npm run test              # runs a script defined in package.json

Common follow-ups: What's the difference between 'dependencies' and 'devDependencies' in package.json, and why does that distinction matter for production deployments?;How does package-lock.json ensure that every developer and deployment environment installs the exact same dependency versions?