1.2 JS Engine and Runtime

What is a JavaScript Engine?

A JavaScript engine is a program that reads, analyzes, and executes JavaScript code. Each browser has its own built-in JavaScript engine.

Engine	Browser/Environment	Developer
V8	Chrome, Edge, Node.js, Deno	Google
SpiderMonkey	Firefox	Mozilla
JavaScriptCore (Nitro)	Safari, WebKit	Apple
Hermes	React Native	Meta

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How JS Engines Work

Let's look at the process of executing JavaScript code step by step.

Step 1: Parsing Source Code

The engine reads text JavaScript code and splits it into tokens.

// This code
const x = 10 + 20;

// Is split into these tokens
// [const] [x] [=] [10] [+] [20] [;]

Step 2: Building the AST

Tokens are transformed into a tree structure called an AST (Abstract Syntax Tree).

// AST for const x = 10 + 20; (simplified)
{
  type: "VariableDeclaration",
  kind: "const",
  declarations: [{
    type: "VariableDeclarator",
    id: { type: "Identifier", name: "x" },
    init: {
      type: "BinaryExpression",
      operator: "+",
      left: { type: "Literal", value: 10 },
      right: { type: "Literal", value: 20 }
    }
  }]
}

Step 3: Bytecode Generation (Interpreter)

The AST is converted into bytecode. Bytecode is higher-level than machine code but executes faster than source code.

In the V8 engine, the Ignition interpreter handles this step.

Step 4: JIT Compilation

When frequently executed code (Hot Code) is detected, the JIT (Just-In-Time) compiler compiles it to machine code to improve performance.

In V8, the TurboFan optimizing compiler handles this.

Source Code
    ↓  Parsing
   AST
    ↓  Compilation
Bytecode  ←→  Executed by Ignition interpreter
    ↓  Hot Code detected
Optimized Machine Code  ←  JIT compiled by TurboFan

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V8 Engine Deep Dive

V8 is Google's high-performance JavaScript engine, used in Chrome and Node.js.

V8 Memory Structure

┌─────────────────────────────────┐
│           V8 Heap Memory        │
├─────────────────────────────────┤
│  New Space (Young Generation)   │ ← Newly created objects
│  (1~8MB)                        │
├─────────────────────────────────┤
│  Old Space (Old Generation)     │ ← Long-lived objects
│  (default max ~1.5GB)           │
├─────────────────────────────────┤
│  Code Space                     │ ← Compiled code
├─────────────────────────────────┤
│  Large Object Space             │ ← Large objects (>512KB)
└─────────────────────────────────┘

Garbage Collection

V8 uses the Mark-and-Sweep algorithm to automatically free memory for objects that are no longer referenced.

function createData() {
  // After function runs, these objects have no references → GC targets
  const bigArray = new Array(1000000).fill(0);
  const obj = { data: bigArray };
  return obj.data[0]; // Only returns the number 0
} // bigArray, obj freed by GC

let result = createData(); // 0
// result = 0, and bigArray/obj already freed

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Browser Runtime

The JavaScript engine alone cannot manipulate the DOM or make HTTP requests. It works together with Web APIs provided by the browser.

Browser Runtime Components

┌─────────────────────────────────────────────────┐
│                   Browser                        │
│  ┌─────────────────┐  ┌────────────────────────┐ │
│  │   JavaScript    │  │       Web APIs         │ │
│  │     Engine      │  │  - DOM                 │ │
│  │  (V8/Spider     │  │  - Fetch API           │ │
│  │   Monkey etc)   │  │  - setTimeout          │ │
│  │                 │  │  - Canvas API          │ │
│  │  ┌───────────┐  │  │  - Web Storage         │ │
│  │  │ Call Stack│  │  │  - WebSocket           │ │
│  │  └───────────┘  │  └────────────────────────┘ │
│  └─────────────────┘                             │
│  ┌─────────────────────────────────────────────┐ │
│  │           Event Loop + Callback Queue        │ │
│  └─────────────────────────────────────────────┘ │
└─────────────────────────────────────────────────┘

Browser-Only Global Objects

// window object (global object in browsers)
console.log(window.innerWidth);  // Browser window width
console.log(window.location.href); // Current URL

// document object (DOM access)
const btn = document.querySelector('#myBtn');
btn.addEventListener('click', () => alert('Clicked!'));

// navigator object (browser/device info)
console.log(navigator.userAgent);
console.log(navigator.language); // 'en-US'

// history object (browser history)
history.back();
history.pushState({}, '', '/new-url');

Event Loop (Brief Introduction)

JavaScript is single-threaded but can handle asynchronous processing. The event loop manages the Call Stack and Callback Queue.

console.log('1. Start');

setTimeout(() => {
  console.log('3. setTimeout callback (after 1 second)');
}, 1000);

console.log('2. End');

// Output order:
// 1. Start
// 2. End
// 3. setTimeout callback (after 1 second)

Event loop details are covered in depth in Ch6.

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Node.js Runtime

Node.js is a runtime environment that allows JavaScript to run outside of browsers. Ryan Dahl built it on top of the V8 engine in 2009.

Node.js Components

┌─────────────────────────────────────────┐
│                Node.js                  │
│  ┌───────────────┐  ┌─────────────────┐ │
│  │   V8 Engine   │  │  Node.js APIs   │ │
│  │               │  │  - fs (files)   │ │
│  │  JavaScript   │  │  - http         │ │
│  │  Execution    │  │  - path         │ │
│  └───────────────┘  │  - os           │ │
│                     │  - crypto       │ │
│  ┌───────────────┐  └─────────────────┘ │
│  │    libuv      │                      │
│  │  (Event Loop  │                      │
│  │   + I/O)      │                      │
│  └───────────────┘                      │
└─────────────────────────────────────────┘

Node.js-Only Features

// File system access (not available in browsers)
import { readFileSync, writeFileSync } from 'fs';

const content = readFileSync('./data.txt', 'utf-8');
console.log(content);

writeFileSync('./output.txt', 'Hello Node.js!');

// HTTP server (not available in browsers)
import { createServer } from 'http';

const server = createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('Hello World!');
});

server.listen(3000, () => console.log('Server running: http://localhost:3000'));

// process object (Node.js global)
console.log(process.version);    // Node.js version
console.log(process.platform);   // 'win32', 'linux', 'darwin'
console.log(process.env.PATH);   // Environment variables

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Browser vs Node.js Comparison

Item	Browser	Node.js
Global Object	window	global, globalThis
Module System	ESM (import/export)	ESM + CJS (require)
DOM	Yes	No
File System	No	Yes
HTTP Server	No	Yes
Native Modules	Web APIs	Node.js APIs
Sandbox	Yes (security restricted)	No (unrestricted)

Environment Detection with globalThis

// Works in both browser and Node.js
if (typeof window !== 'undefined') {
  console.log('Browser environment');
  console.log(window.location.href);
} else {
  console.log('Node.js environment');
  console.log(process.version);
}

// Better approach: use globalThis
console.log(globalThis); // Browser: window, Node.js: global

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Pro Tips

Coding Patterns for V8 Optimization

V8 uses Hidden Classes and Inline Cache techniques to optimize object access:

// Good: always initialize properties in the same order
class Point {
  constructor(x, y) {
    this.x = x; // always x first
    this.y = y; // always y second
  }
}

// Bad: dynamically adding properties (changes Hidden Class)
const p1 = {};
p1.x = 1;
p1.y = 2; // Hidden Class change → de-optimization

// Maintain numeric types (V8 uses internal representations: Smi, Double, Object)
const arr = [1, 2, 3];     // Smi array (fast)
arr.push('text');          // Changes to Object array (slower)
arr.push(1.5);             // Changes to Double array