Do modern JavaScript engines optimize away unused closed-over variables?
Some do. If a closure does not reference a variable, the engine may free it even though it is in the same lexical environment. But this is an optimization, not guaranteed across all engines. Do not rely on it; extract only what you need.
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More FAQs in Closures and Memory Leaks in JavaScript
A closure holds a reference to its lexical environment. As long as the closure is reachable, the environment (and all its variables) cannot be garbage collected. If closures accumulate or keep large objects alive, memory grows unbounded.
Accumulating handlers in arrays, forgotten timers (setInterval never cleared), event listeners on detached DOM elements, unbounded caches of closures, and accidental closures that keep large objects alive without using them.
Remove event listeners when done, clear timers with clearInterval/clearTimeout, use WeakMap or WeakRef for weak references, bound cache size (LRU), and extract only what you need instead of closing over large objects.
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