What Makes JS Run? — The JavaScript Runtime Environment (JRE)

JS needs a special environment to run — this is called the JavaScript Runtime Environment (JRE).

Think of JRE as a Kitchen:

JS Engine: The chef who cooks your code.
Web APIs: Extra tools like microwave, oven, or fridge (things like DOM, setTimeout, etc.).
Event Loop: The kitchen manager who decides when things get cooked.
Callback & Microtask Queue: Waiting lines for dishes to be cooked.

💡 Important: Browsers (like Chrome, Firefox) and some platforms (like Node.js) provide this environment so that your JS code can work.

📜 ECMAScript & JavaScript Engines

ECMAScript: It's like a rulebook (standard) that every JS engine follows.
JavaScript Engine: Software that understands and runs your JS code.

Different browsers use different JS engines:

Chrome: V8
Firefox: SpiderMonkey
Safari: JavaScriptCore
Edge: Now uses V8 (used to have Chakra)

What is a JavaScript Engine?

A JS Engine is not a machine. It’s a program written in low-level languages like C++. Its job is to take your JS code and turn it into something your computer understands — machine code.

Let’s understand what happens inside.

🔄 The 3 Stages of Code Inside JS Engine

When you write JavaScript like:

let a = 5;

The engine processes this in 3 steps:

1. Parsing:

Breaks code into tokens (small parts): let, a, =, 5.
Creates an AST (Abstract Syntax Tree) – a tree structure that represents your code.

You can try this yourself at astexplorer.net

2. Compilation (Yes, JS Compiles!)

JavaScript uses something called JIT (Just-In-Time) Compilation, which is a mix of:

Interpreter: Quickly runs the code.
Compiler: Optimizes it while it runs.

✅ So yes, JavaScript is compiled! It's not just interpreted like in the old days.

👉 The engine compiles and executes at the same time to improve speed.

Example:

function add(a, b) {
  return a + b;
}
add(2, 3);

First, interpreter runs it.
If the function is used often, compiler will optimize it on the go!

Extra Reading:

3. Execution:

Once the code is compiled, it's ready to run.

Two important parts during execution:

Memory Heap: Where all variables and objects live.
Call Stack: Keeps track of function calls (Remember this from async JS?).

👮‍♂️ Garbage Collector: Cleans up unused memory using a method called Mark and Sweep (marks unused variables and removes them).

V8 Engine – Behind the Scenes

The V8 Engine (used in Chrome & Node.js) has:

Ignition – Interpreter
TurboFan – Optimizing Compiler
Orinoco – Garbage Collector

This combo helps JS run fast and efficiently.

Want to See This Live?

Watch this full explanation on YouTube by Akshay Saini:

🔗 JS Engine Exposed – Akshay Saini

📝 Final Notes

JavaScript Engines have evolved a lot — from simple interpreters to smart compilers.
JS is now a JIT compiled language (Best of both worlds).
Knowing how your code runs helps you write better and faster applications.

JS Engine Exposed – Google's V8 Architecture (Made Simple!)

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