How JavaScript Engines Work

A JavaScript engine is a program that reads, compiles, and executes JavaScript code. It is designed to parse and understand source code, optimize it for fast execution, manage memory and garbage collection, and communicate with the browser engine and rendering engine to update the DOM or CSSOM.

Understanding how JavaScript engines work helps you write more performant code and understand why certain patterns are faster than others.

Popular JavaScript Engines

Different browsers use different JavaScript engines, each with its own optimizations and features.

All modern engines use Just-In-Time compilation and advanced optimization techniques to make JavaScript execution fast.

Engine Architecture

Every modern JavaScript engine consists of several tightly connected subsystems. The pipeline looks like this:

Source Code
  ↓
Parser → AST → Interpreter → JIT Compiler → Machine Code → Execution

Let’s break down each part in detail.

Parser (Syntax Analyzer)

The parser converts JavaScript source code into an Abstract Syntax Tree. This happens in two phases: lexical analysis and parsing.

Lexical Analysis (Tokenization)

Breaks source code into tokens: keywords, identifiers, symbols.

let x = 5;

This becomes tokens: let, x, =, 5, ;

Parsing (Grammar Analysis)

Verifies syntax and builds the AST.

VariableDeclaration
├── Identifier: x
└── Literal: 5

If syntax errors are found, parsing halts immediately.

Abstract Syntax Tree

The AST is an internal tree-like representation of JavaScript code. Each node corresponds to a syntactic construct like variables, functions, or loops.

function greet(name) {
  return "Hello " + name;
}

This creates an AST structure:

FunctionDeclaration
├── Identifier: greet
└── BlockStatement
    └── ReturnStatement
        └── BinaryExpression (+)
            ├── Literal: "Hello "
            └── Identifier: name

The AST is crucial for further stages including optimization and execution.

Interpreter (Bytecode Generator)

The interpreter translates the AST into bytecode, a lower-level portable representation. It executes this bytecode immediately for fast startup.

let a = 3 + 4;

Becomes bytecode (simplified):

LoadConst 3
LoadConst 4
Add
Store a

V8 uses an interpreter called Ignition that quickly interprets JavaScript into bytecode before optimization.

JIT Compiler (Just-In-Time Compiler)

The JIT compiler converts hot (frequently executed) parts of the bytecode into machine code for speed.

The Process

1. Interpreter runs JavaScript initially
2. The engine monitors which functions or loops are executed repeatedly
3. Hot sections are handed to the JIT compiler
4. The JIT compiles them into optimized native machine code for the CPU
5. Execution switches to the compiled code for better performance

Example Engines

Profiler and Deoptimizer

The JIT collects runtime type information, such as variable types and function call frequency. If assumptions become invalid, like a variable changing type, the JIT deoptimizes back to bytecode.

This dynamic behavior allows JavaScript engines to stay both fast and flexible.

Garbage Collector (Memory Manager)

JavaScript uses automatic garbage collection to manage memory by reclaiming space no longer used.

Process

JavaScript uses mark-and-sweep:

1. Mark phase: Engine marks all objects reachable from roots like the global object or current scope
2. Sweep phase: Removes unreferenced objects

function run() {
  let obj = { name: "John" };
}
run();
// After function exits, obj is no longer reachable and gets garbage collected

Heap and Stack Memory

When the stack unwinds, variables go out of scope, but heap objects persist until garbage collection cleans them up.

Execution Context and Call Stack

Every JavaScript execution happens inside an execution context, which contains a variable environment, lexical environment (scope), and this binding.

These contexts are stored in the call stack:

Global Context
 ├── Function A()
 │     └── Function B()

The stack grows and shrinks as functions are called and returned.

Engine Pipeline Summary

JavaScript Source Code
   ↓
Lexical Analysis → Tokens
   ↓
Parser → AST
   ↓
Interpreter → Bytecode
   ↓
JIT Compiler → Machine Code
   ↓
Execution → Memory Management

Modern Optimizations

Modern JavaScript engines use several optimization techniques:

1. Hidden Classes and Inline Caches: Speeds up property access by predicting object structure

2. Speculative Optimization: Assumes variable types and recompiles if type changes

3. Lazy Parsing: Only parses functions when needed, reducing load time

4. Concurrent Garbage Collection: GC runs in parallel with execution to reduce pauses

5. WebAssembly Integration: Executes precompiled binary code alongside JavaScript for performance-critical tasks

6. Multi-threaded Compilation: Compiles code in parallel to reduce latency

How the Engine Interacts with the Browser

User clicks button
 ↓
Browser Engine → JS Engine executes onClick() handler
 ↓
Function modifies DOM (via Rendering Engine)
 ↓
Rendering Engine updates Render Tree → Layout → Paint
 ↓
User sees updated content

Summary

Understanding the JavaScript engine helps you write code that takes advantage of these optimizations. Consistent object shapes, avoiding type changes, and understanding how hot code paths work all contribute to better performance.