Functions, Scope & Object Model

Memory, References & Garbage Collection

JavaScript automatically frees values that are no longer reachable, but automatic garbage collection does not prevent leaks. A leak is unneeded data that remains reachable.

Reachability determines collection
References make mutation visible
Closures can keep data alive
Caches need eviction
Event listeners need cleanup

Reachability

The large object stays alive because a root can still reach it through the callback.

Unbounded cache smell

const cache = new Map()

function remember(key, value) {
  cache.set(key, value)
}

// Unless entries are removed, this grows for the lifetime of the process/page.

A cache without eviction is just a leak with better marketing.

Common leak sources
Source	Why it leaks	Mitigation
Global Map cache	Entries never removed	TTL/size limit/clear
Event listener	Callback retained	remove listener / abort signal
Timer/interval	Callback retained	clear timeout/interval
Closure	Captures large object	Capture less / cleanup
Detached DOM	JS still references node	Release references