The JavaScript Debugging Stories From Real PRs
Four solutions to the same closure-and-var trap, plus one bug that took me an afternoon. 5 questions, all from real PRs where the test passed and the user-visible behavior was still wrong.
Question Bundle
JavaScript
interview-prep
js-language
closures
js-this
By @marcusreddy
December 30, 2025
·
Updated May 18, 2026
446 views
10
4.4 (9)
First time I saw this in a PR I assumed setTimeout was broken. It logs 5,5,5,5,5 instead of 0,1,2,3,4. Walk through why, without writing the fix yet.
What I want to see
The diagnosis: var is function-scoped, the loop runs to completion before any timer fires, and each callback closes over the same i (now 5). The fix is in q-2 through q-4, not here.
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