Time: 0:00
Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).
This bug belongs to the logic errors pattern
The wrong operator, an inverted condition, a formula the right way round but backwards.
What went wrong: When the item is at index 0, idx is 0, which is falsy in if (idx). Furthermore, if not found, idx is -1, which is truthy.
The fix: Compare explicitly against -1: return idx !== -1
Remember: In JavaScript, findIndex() returns -1 when missing; never check findIndex results with truthy if (idx).
The pattern behind it: logic errors.
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