TypeScript · Senior · Question 67 of 75

Why can a `let` variable lose its narrowing inside a callback but a `const` keeps it?

✓Correct answer: A let may be reassigned before the callback runs, so TS can't trust the earlier check; const can't change
2Callbacks reset all types
3const variables are never narrowed

Explanation

Recent versions preserve narrowing for let in closures created after the last assignment, but copying into a const is the simple fix.

Further reading

TS Handbook: Narrowing ↗

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