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Mark-and-Sweep

Mark-and-Sweep

Introduction

Mark-and-sweep is the fundamental algorithm JavaScript engines use for garbage collection. It works in two phases: mark (find reachable objects) and sweep (remove unreachable ones).

How It Works

flowchart TD
    A["Memory full of objects"]
    B["Mark Phase: Start from roots,\ntraverse all references,\nmark reachable objects"]
    C["Sweep Phase: Remove\nunmarked (unreachable) objects"]
    D["Memory freed for new allocations"]

    A --> B
    B --> C
    C --> D

The Mark Phase

The GC starts from roots — objects that are always reachable:

The global object (window/globalThis)
Local variables in the current function
Parameters
The call stack

From these roots, the GC traverses all references, marking every object it can reach.

The Sweep Phase

Any object not marked during the traversal is considered unreachable and its memory is freed.

// Reachable objects (marked — kept alive)
const globalData = { value: 'important' };

function process() {
  const local = { nested: { deep: 'data' } };
  // local, nested, deep are all reachable during execution
  return local.nested.deep;
}
// After return, local is unreachable → GC can collect it

What Stops Mark-and-Sweep?

let user = { name: 'Alice' };
let admin = user; // Both reference the same object

user = null; // Object is still reachable via 'admin'
// GC cannot collect it yet

admin = null; // Now unreachable → eligible for collection

Visual Example

// Objects and their references:
const root = {
  child1: { data: 'A' },
  child2: { data: 'B' }
};
// root → child1 ✅
// root → child2 ✅

root.child1 = null;
// child1 is still reachable?
// No! root.child1 = null removes the reference
// child1 { data: 'A' } is now unreachable → SWEEP

// child2 is still reachable via root.child2 → MARK kept

Summary

Roots: global object, local variables, call stack
Mark phase: traverse from roots, mark reachable objects
Sweep phase: free unmarked objects
Only unreachable objects are collected
This is the foundation all modern JS GC builds on

Further Reading