Garbage Collection in V8 and Node.js Explained

Garbage Collection in V8 and Node.js Explained

V8's garbage collector frees memory automatically. Here is how it works and how to avoid leaks in Node.js.

What Garbage Collection Does

The garbage collector finds objects that are no longer reachable from the running code and frees their memory. This is automatic; you do not manually free memory in JavaScript.

V8's Generational GC

V8 uses a generational garbage collector. New objects are in the young generation, which is collected often (minor GC). Long-lived objects move to the old generation, which is collected less often (major GC).

Minor vs Major GC

Minor GC (scavenge) collects the young generation, which is fast and frequent. Major GC (mark-sweep or mark-compact) collects the old generation, which is slower and less frequent. This split makes GC efficient.

How Memory Leaks Happen in Node.js

Despite GC, leaks happen when objects are still referenced unintentionally. A forgotten event listener keeping an object alive, a closure capturing more than it needs, or a global cache growing without bounds.

Detecting Memory Leaks

Use the Node.js inspector and heap snapshots to find leaks. Take a snapshot, run your code, take another, and compare. Growing objects between snapshots point to the leak.

Common Leak Patterns

Forgotten event listeners (always remove them on cleanup), global caches that grow without bounds, and closures that capture large objects unnecessarily. Fix these by cleaning up listeners and bounding caches.

The Takeaway

V8's generational GC frees memory automatically: young objects often, old objects less often. Leaks happen from forgotten references like event listeners and unbounded caches. Detect leaks with heap snapshots.