Garbage Collection

In short: JavaScript's garbage collector reclaims memory for anything that is no longer reachable — not tied, by any chain of references, back to a root like a global variable or a currently-running function's local variables. Reachable, however many steps away, means kept; unreachable means eligible for collection. You can never observe or force the exact moment collection happens — only reachability is something your code actually controls.

Reachable through a chain, however long

An object stays alive as long as something reachable points to it — directly, or through other objects:

Setting session to null doesn't make { name: "Asha" } unreachable — savedUser still points straight at it. An object needs every path to it cut before it becomes eligible for collection, not just one.

WeakMap: a reference that doesn't count

Sometimes you want to attach data to an object without that attachment itself keeping the object alive forever. That's what WeakMap is for — verified here only by what you can observe (the entry exists while the key does), never by timing:

A regular Map would hold a strong reference to obj as a key — the Map itself would keep obj reachable forever, even if nothing else in the program still needed it. WeakMap holds its keys weakly: that reference doesn't count toward reachability. Once every other reference to obj is gone, it becomes eligible for collection — and its entry in the WeakMap disappears along with it. Exactly when that happens is decided by the engine; nothing here claims a specific moment, because that part genuinely isn't observable from code.

Quick practice