If you’re learning JavaScript, you’ve probably wondered what’s really happening when your code runs. What does the browser do with your console.log()? How does JavaScript know what to execute — and when? This post will break it all down in a simple way.

We’ll answer the big question: How JavaScript works behind the scenes

What Is JavaScript?

JavaScript is a high-level, interpreted programming language. It’s mostly used in web development to add interactivity — like animations, popups, and dynamic content — to websites.

It runs in the browser (like Chrome or Firefox) using something called a JavaScript engine. Every major browser has its own engine:

• Chrome uses V8
• Firefox uses SpiderMonkey
• Safari uses JavaScriptCore

JavaScript Is Single-Threaded

This is one of the most important things to understand.

JavaScript can only do one thing at a time. That’s what “single-threaded” means.

Think of it like a to-do list. JavaScript goes through it from top to bottom, step by step. If one item takes a while, the rest have to wait. This is why asynchronous code is so important in JavaScript — but we’ll get to that shortly.

The JavaScript Engine

So, how does JavaScript actually run?

JavaScript code runs in a specific process inside the browser. Here’s the simplified flow:

1. Parsing: The engine reads your code and turns it into a structure called an Abstract Syntax Tree (AST).
2. Compilation: JavaScript is compiled using something called Just-In-Time (JIT) compilation.
3. Execution: The compiled code is then executed, line by line.

Let’s walk through a basic example.

function greet() {
  console.log("Hello!");
}

greet();

What’s going on here:

• The engine sees the greet function and stores it.
• When it reaches greet(), it calls the function.
• Inside the function, it runs console.log("Hello!") and prints it to the console.

Behind the scenes, the engine has parsed, compiled, and executed all of this.

JavaScript Building Blocks

To understand how JavaScript works, you need to know its foundational components:

1. Memory Heap

This is where JavaScript stores data like variables, objects, and functions. It’s a general-purpose memory pool.

let user = { name: "amol" };

Here, the object { name: "amol" } is stored in the heap.

2. Call Stack

This tracks which functions are currently being run. Think of it like a stack of dishes — you add one on top, and remove the top one when it’s done.

function sayHi() {
  console.log("Hi");
}

function start() {
  sayHi();
}

start();

Call stack process:

1. start() is added to the stack.
2. Inside it, sayHi() is added.
3. Inside sayHi, console.log() is added.
4. As each function finishes, it’s removed from the stack.

If something keeps calling itself and never finishes, the stack overflows.

How JavaScript Handles Asynchronous Code

Even though JavaScript runs one thing at a time, it can still handle tasks that take time — like waiting for a server response or a timer.

So how does JavaScript stay non-blocking?

The Event Loop

This is the key component that manages async operations.

Here’s how it works:

• The browser provides Web APIs like setTimeout or fetch.
• Once an async task finishes, its callback is placed into a queue.
• The event loop keeps checking if the call stack is empty. If it is, it pulls the next task from the queue and runs it.

console.log("Start");

setTimeout(() => {
  console.log("Timeout finished");
}, 2000);
console.log("End");

Output:

Start
End
Timeout finished

Even though setTimeout appears in the middle of the code, it runs last. Why? Because it’s asynchronous and handled by the browser in the background.

Behind the Scenes: JavaScript Runtime Environment

So far we’ve talked about engines. But there’s more to the story.

The JavaScript Runtime includes:

• The JavaScript engine (like V8)
• Web APIs (like setTimeout, DOM, fetch)
• Callback queue (tasks waiting to be run)
• Event loop (decides what runs next)

JavaScript itself doesn’t know how to wait or handle time. The browser (or Node.js) provides those abilities.

Full Example: Microtasks vs Macrotasks

Let’s look at how JavaScript prioritizes tasks.

console.log("1");

setTimeout(() => {
  console.log("2");
}, 0);
Promise.resolve().then(() => {
  console.log("3");
});
console.log("4");

Output:

1
4
3
2

Why?

• console.log("1") and console.log("4") run immediately.
• Promise.then() uses the microtask queue, which runs before the callback queue.
• setTimeout goes into the callback queue (macrotask) and waits its turn.

This shows how JavaScript prioritizes tasks — synchronous first, then microtasks (promises), then callback tasks (timeouts, events). The event loop gives microtasks priority over macrotasks.

Final Thoughts: How JavaScript Works

Understanding how JavaScript works under the hood helps you write better, faster, and safer code. Here’s a quick recap:

• JavaScript runs one thing at a time (single-threaded).
• It uses a call stack to track function calls.
• Async tasks are handled using Web APIs, callback queues, and the event loop.
• Modern engines use JIT compilation for speed.
• Knowing how memory and the stack work helps prevent bugs and optimize performance.

FAQs

Q: Why is JavaScript single-threaded?
 Because it was originally designed for simple web interactions. A single thread avoids complex race conditions and keeps the user experience smooth.

Q: What’s the difference between microtasks and macrotasks?
 Microtasks (like promises) run before macrotasks (like setTimeout). They go into different queues, and the event loop checks microtasks first.

Q: Is JavaScript the same in Node.js and browsers?
 The language is the same, but the runtime environment is different. Node.js uses V8 too, but its APIs are server-focused.

Conclusion

Now that you understand how JavaScript works behind the scenes, you’ll find it easier to debug, optimize, and write more efficient code. Whether you’re working in the browser or with Node.js, these principles are the foundation of how JavaScript runs your code.

If this helped clear things up, share it with a friend who’s still puzzled by setTimeout() delays and stack overflows 😉