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Shruti Kumbhar
Shruti Kumbhar

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Understanding JavaScript Engine Internals: V8, SpiderMonkey, and More

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JavaScript is a high-level, interpreted language that has evolved to deliver near-native performance, thanks to sophisticated engines like V8, SpiderMonkey, JavaScriptCore, and Chakra. This article explores the internal workings of these engines and how they achieve efficient execution.

What is a JavaScript Engine?

A JavaScript engine is a program that executes JavaScript code. Most engines today are built with similar core components:

  • Parser: Transforms source code into an Abstract Syntax Tree (AST)
  • Interpreter: Executes the code line-by-line initially
  • JIT Compiler: Compiles hot code paths into optimized machine code
  • Garbage Collector: Reclaims memory no longer in use

Architecture Overview

This image shows the common architecture shared by most modern JS engines.

Key JavaScript Engines

Engine Browser Language Creator
V8 Chrome, Node.js C++ Google
SpiderMonkey Firefox C++ Mozilla
JavaScriptCore Safari C++ Apple
Chakra Edge (Legacy) C++ Microsoft

V8 Engine: Under the Hood

Workflow

  1. Parsing: JavaScript is parsed into an AST.
  2. Ignition (Interpreter): Executes unoptimized bytecode quickly.
  3. Turbofan (JIT Compiler): Optimizes frequently executed code into fast machine code.

Example 

function square(n) {
  return n * n;
}

console.log(square(5)); // 25
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  • Initially run by Ignition
  • Optimized by Turbofan if called repeatedly

SpiderMonkey Engine

SpiderMonkey is the JS engine developed by Mozilla for Firefox.

Components

  • Parser: Creates AST
  • Interpreter: Baseline compiler for quick start
  • IonMonkey (JIT): Aggressively optimizes hot code paths
  • Garbage Collector: Handles memory management

Memory Management and Garbage Collection

Modern engines use generational garbage collection:

  • Young Generation: For short-lived objects (e.g., function-scoped variables)
  • Old Generation: For longer-lived data

Example:

function createGarbage() {
  let arr = [];
  for (let i = 0; i < 100000; i++) {
    arr.push({ index: i });
  }
}
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This creates many short-lived objects that are collected after execution.

Performance Optimizations

Inline Caching

Caches object property lookups for repeated accesses.

function getName(user) {
  return user.name;
}
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If user consistently has the same structure, the engine caches the property access.

Hidden Classes (V8)

Objects get assigned hidden classes to speed up property access:

let obj = { a: 1 };  // HiddenClass1
obj.b = 2;           // HiddenClass2
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Changing object structure dynamically can harm performance.

Interpreter vs JIT: Trade-offs

Component Speed Best For
Interpreter Fast Start Short-lived code
JIT Compiler High Perf Long-lived code

Engines mix both for optimal performance.

Real-World Use: Node.js and V8

Node.js uses V8 to execute server-side JavaScript:

const fs = require('fs');

setInterval(() => {
  fs.readFile('file.txt', () => {
    console.log('Read complete');
  });
}, 1000);
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  • V8 runs JS
  • Node offloads I/O to libuv

Component and Role

Component Role
Parser Converts JS to AST
Interpreter Executes code line-by-line
JIT Compiler Converts hot code to machine code
Garbage Collector Reclaims unused memory

References

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