Memory and garbage collection

Stack and heap in plain words

While a function runs, its small local values live in a fast, short-lived area called the stack; they vanish when the function returns. Objects, arrays, closures and strings live in a larger area called the heap, where they stay as long as something refers to them.

Garbage collection and reachability

You never free memory yourself in JavaScript. The engine's garbage collector (GC) regularly finds values that are no longer reachable, meaning no chain of references leads to them from the "roots" (global variables, the current call stack, active closures), and reuses their memory.

let user = { name: 'Asha', photo: new Array(1e6).fill(0) };
user = null;   // the big object is now unreachable → the GC can free it

const cache = [];
function remember(item) { cache.push(item); }   // everything pushed stays reachable forever

Common leaks

// Leaky: a "recent searches" cache that grows forever on a server
const recent = new Map();
function track(userId, query) {
  recent.set(`${userId}:${Date.now()}`, query);   // never deleted
}

// Fixed: keep only the newest 100 entries
const MAX = 100;
function trackSafe(userId, query) {
  recent.set(`${userId}:${Date.now()}`, query);
  if (recent.size > MAX) recent.delete(recent.keys().next().value);  // oldest first
}

WeakMap and WeakRef

A WeakMap keeps a value attached to an object key without keeping that key alive. When the object is no longer used elsewhere, its entry disappears automatically: perfect for "extra data about a DOM element or request object". WeakRef and FinalizationRegistry exist for advanced cases, but their timing is unpredictable, so avoid them unless you really need them.

Finding leaks

1. Watch memory over time: in Node, process.memoryUsage().heapUsed; in the browser, the Memory panel of DevTools.
2. Repeat an action many times (open and close a dialog 50 times). If memory keeps growing and never drops, something leaks.
3. Take two heap snapshots (before and after) and compare which objects grew. For Node, run node --inspect and open chrome://inspect.

setInterval(() => {
  const mb = (process.memoryUsage().heapUsed / 1024 / 1024).toFixed(1);
  console.log('heap used:', mb, 'MB');
}, 5000);