Use const by default and let for reassignable variables. Keep variables in the tightest scope possible. Use let in classic for loops, const in for...of and for...in. Wrap switch cases in braces. Avoid shadowing. Do not close over large objects unnecessarily.
Tighter scope means fewer bugs (variables are not visible where they are not needed), better garbage collection (variables are freed sooner), and more readable code (intent is clear).
Wrap each case in braces to give it its own block scope. Without braces, all cases share one scope, and re-declaring let or const with the same name throws SyntaxError.
Do not close over large objects unnecessarily. Extract what you need (e.g., const len = big.length; return () => len;) instead of closing over the whole object. Remove handlers when no longer needed.
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