Learn Node.js

What Is Node.js?

Node.js is an open-source, cross-platform JavaScript runtime built on Google's V8 engine, the same engine that powers Chrome. Released in 2009 by Ryan Dahl, it lets developers execute JavaScript on a server, a desktop, or the command line instead of only inside a browser tab. Because it shares one language with the front end, teams can move code and skills between client and server with almost no friction.

Why Developers Use Node.js

• Full-stack JavaScript — write the browser and the server in one language.
• A single-threaded event loop that serves thousands of concurrent clients without spawning a thread per request.
• npm, the world's largest package registry, giving instant access to reusable modules.
• A small memory footprint and fast startup, which suits microservices, CLIs, and real-time apps.

The Event Loop and Non-Blocking I/O

Node.js runs your JavaScript on a single main thread, but it never waits idly for slow operations like disk reads or network calls. Those tasks are handed off to the operating system or a background thread pool (via a library called libuv), and Node keeps executing other code. When the operation finishes, its callback is queued and the event loop picks it up once the current work is done.

console.log("1: script start");

setTimeout(() => {
  console.log("2: timeout callback");
}, 0);

Promise.resolve().then(() => {
  console.log("3: promise callback");
});

console.log("4: script end");

// Actual output order: 1, 4, 3, 2

Blocking vs. Non-Blocking I/O

function blockFor(ms) {
  const end = Date.now() + ms;
  while (Date.now() < end) {
    // busy-wait: this loop occupies the single thread completely
  }
}

console.log("Start");
setTimeout(() => console.log("Delayed timer fires late"), 100);
blockFor(2000); // freezes the entire process for 2 seconds
console.log("End of synchronous work");

// The timer set for 100ms doesn't fire until after blockFor() finishes,
// because nothing else can run while the thread is busy.

console.log("Task A: begins");

setTimeout(() => {
  console.log("Task B: runs later, without blocking Task C");
}, 500);

console.log("Task C: runs immediately, right after Task A");

// Output: Task A, Task C, Task B