How Garbage Collection Works in JavaScript

By Rahul — Google Frontend Engineer

The Basics

JavaScript automatically manages memory. When you create objects, arrays, or strings, memory is allocated. When they are no longer reachable, the garbage collector frees the memory. You do not call free() like in C — but you can still cause memory leaks.

Reachability

The GC starts from "roots" (global object, currently executing function's variables, closures) and marks everything reachable from them. Anything not reachable is garbage.

Mark-and-Sweep (Modern V8)

1. Mark phase: Starting from roots, mark all reachable objects
2. Sweep phase: Free memory of all unmarked objects
3. Compact: Move remaining objects together to reduce fragmentation

Generational Collection in V8

V8 divides memory into two areas:

• Young Generation (1-8MB): Newly created objects. Collected frequently with a fast "Scavenger". Most objects die young
• Old Generation (up to GBs): Objects that survived multiple young-gen collections. Collected less frequently with Mark-Sweep-Compact

Common Memory Leak Patterns

Forgotten Timers

Detached DOM Nodes

Closures Holding Large Data

WeakRef and FinalizationRegistry

Debugging Memory Leaks

• Chrome DevTools → Memory tab → Take Heap Snapshot
• Compare snapshots before and after a suspected leak
• Look for growing "Detached DOM trees" and increasing object counts
• Use performance.measureUserAgentSpecificMemory() for production monitoring

Summary

V8 uses generational mark-and-sweep garbage collection. Most objects die young. Memory leaks happen from forgotten timers, detached DOM nodes, and unbounded caches. Use Chrome DevTools Memory tab to find leaks. Use WeakMap/WeakSet for caches that should not prevent GC.