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