How does garbage collection work and what causes memory leaks?

Modern engines use mark-and-sweep. Starting from roots, the collector marks everything reachable, then frees the unmarked objects. You cannot force collection or free memory manually; you can only remove references.

Common leak patterns

• Global variables that keep large objects alive
• Timers or intervals that are never cleared
• Event listeners on long-lived elements that are never removed
• Detached DOM nodes retained by a JavaScript reference
• Closures that capture more data than the callback needs
• Unbounded caches that never evict entries

function attach(element) {
  const handler = () => console.log("clicked");
  element.addEventListener("click", handler);

  return () => element.removeEventListener("click", handler);
}

A closure itself is not a leak. It becomes one when a long-lived callback keeps a large object reachable after the feature is gone. WeakMap and WeakSet help when metadata should not extend an object's lifetime.

const cache = new WeakMap();

function store(node, data) {
  cache.set(node, data);
}