Why does let fix the setTimeout closure bug but var does not in JavaScript?
let is block-scoped and creates a fresh binding per iteration. Each callback closes over its own i. var is function-scoped; all callbacks share one i, which is the final value by the time they run.
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More FAQs in Fixing the setTimeout Closure Bug Three Ways in JavaScript
Use let (fresh binding per iteration), use an IIFE (new function scope with pass-by-value parameter), or use setTimeout's extra arguments (pass the current value as an argument to the callback).
let. It is the cleanest and most readable. It creates a fresh binding per iteration without extra nesting or arguments. Use it in all ES6+ code.
The IIFE creates a new function scope per iteration and passes i as a parameter (j). Primitives are passed by value, so j is a copy of i at that iteration. The closure captures j, not i.
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