Internals of Google's V8 JS Engine

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Category: JavaScript

Introduction

Being used to JavaScript being flexible and all, you’ll be surprised of what goes underneath the hood upon running a .js script: V8 — Google’s extremely high-performance engine for JavaScript inside Chrome and node.js. Actually, unlike your average interpreter for JavaScript, v8 compiles JavaScript into code that’s native to the executing machine. Its execution speed comes out very effective. In that regard, below is the technical blog on ‘how JavaScript executes internally in an engine called v8.

Internals-of-V8_-How-JavaScript-is-Really-Executed

The V8 Architecture: What’s Inside?

V8 is a Just-In-Time (JIT) compiler-based engine that’s built for speed and efficiency. It has the following components:

  1. Parser: Converts JavaScript code into an Abstract Syntax Tree (AST).
  2. Ignition (Interpreter): Translates the AST into bytecode for initial execution.
  3. TurboFan (Optimizing Compiler): Converts frequently executed code into highly optimized machine code.
  4. Garbage Collector: Manages memory by cleaning up unused objects.

Now, let’s break down how JavaScript goes from source code to execution.

V8 Execution Step by Step

Parsing: Converting the Code into an AST

While you write JavaScript, V8 does not directly execute it. Instead, it first passes it through a parser that converts source code into an Abstract Syntax Tree (AST).

  • Lexical Analysis: breaks the code down into tokens or keywords, variables, operators, etc.
  • Syntax Analysis: takes these tokens and builds up a tree-like structure called AST representing the code logic

Ignition Generates Bytecodes

Instead of executing JavaScript line after line, V8 employs an Ignition interpreter that interprets AST into a bytecode representation. This procedure provides JavaScript execution as more efficient than direct interpretation.

TurboFan Optimization

While the bytecode from Ignition would suffice for fast execution, V8 further optimizes it to ensure high-performance code. Here is how it works through TurboFan:

  • Profiling: V8 observes the hot code, or frequently executed functions.
  • Optimization: TurboFan compiles the hot code into optimized machine code.
  • Deoptimization: If assumptions made during optimization break, V8 falls back to bytecode execution.

Garbage Collection: Managing Memory Efficiently

JavaScript’s dynamic nature means objects are created and discarded frequently. V8’s garbage collector automatically cleans up unused memory through:

  • Scavenge (Young Generation GC): Quickly removes short-lived objects.
  • Mark-and-Sweep (Old Generation GC): Cleans up long-lived objects when needed.

This process ensures memory is efficiently used, preventing memory leaks and slowdowns.

Why V8 is So Fast

V8’s speed comes from:

  • JIT Compilation: Translates JavaScript to optimized machine code instead of interpreting it.
  • Hidden Classes: Optimizes property access by creating internal class structures.
  • Inlining: Removes function call overhead by directly embedding function logic.
  • Efficient Garbage Collection: Reduces memory pressure and keeps execution smooth.

Conclusion

Understanding V8’s internals helps developers write more efficient JavaScript. From parsing to execution, the combination of Ignition, TurboFan, and garbage collection ensures high performance. Next time you run a JavaScript function, remember—it’s not just being executed; it’s being optimized for speed!

What aspect of V8 excites you the most?

Let’s discuss in the comments!

Happy Coding!!!

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