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Memory Layout

Memory Layout

Introduction

Understanding how JavaScript engines store data in memory helps you write more performant code.

Stack vs Heap Layout

Memory Area	Stores	Speed	Size
Stack	Primitives, references, function frames	Fast	Small (~1MB)
Heap	Objects, arrays, closures	Slower	Large

Object Layout in V8

// Object in memory (simplified)
const obj = { x: 5, y: 10 };

// Memory (conceptual):
// [Map pointer] → Hidden class (shape)
// [x: 5]
// [y: 10]
// [Property array] → Dynamic properties (if needed)

Pointer Compression

V8 uses pointer compression (32-bit pointers instead of 64-bit) to save memory:

Without compression: Each pointer = 8 bytes
With compression:    Each pointer = 4 bytes  ← 50% savings!

Elements Kinds

V8 tracks array element types for optimization:

[1, 2, 3];         // PACKED_SMI_ELEMENTS (fastest)
[1, 2, true];      // PACKED_DOUBLE_ELEMENTS
[1, 2, 'hello'];   // PACKED_ELEMENTS
[1, , 3];           // HOLEY_ELEMENTS (holes = slower)

Summary

Stack: fast, small, for primitives and references
Heap: slower, large, for objects
Pointer compression saves ~40% memory
Elements kinds affect array performance
Avoid holes in arrays for best performance

Further Reading