JavaScript Garbage Collector: Mark-and-Sweep Explained

Garbage Collector is a built-in mechanism in JavaScript that automatically manages memory. It's responsible for freeing memory occupied by objects that are no longer used in the program.

You don't manage memory manually — JavaScript engine does it for you (e.g., V8 in Chrome/Node.js).

How Does Garbage Collector Work?

JavaScript allocates memory for variables and objects when they're created, and frees it when they're no longer reachable.

Main principle:

Object is considered "garbage" if there are no more references to it.

Example:

let user = { name: "Alice" };
user = null; // Now object can be deleted from memory

Algorithms Used in GC

Mark-and-Sweep

Most common algorithm:

• Marks all objects accessible through variables (root objects).
• All other objects are deleted as "garbage".

In this example object C is unreachable and will be deleted.

Reference Counting

Each object stores a number showing how many references point to it. If counter = 0, object is deleted.

Disadvantage: circular references aren't deleted.

let a = {};
let b = {};
a.ref = b;
b.ref = a;

// Both variables are null, but objects still reference each other
a = null;
b = null; // Potential memory leak!

How to Avoid Memory Leaks?

• Remove unused timers: clearInterval, clearTimeout
• Clear subscriptions, event handlers
• Nullify large objects and DOM references: ref.current = null
• Avoid global variables
• Use WeakMap, WeakSet — they don't prevent garbage collection

Signs of Memory Leaks

• Constant memory growth during use
• Delays, freezes, lags
• High RAM consumption in DevTools → Performance/Memory tab