Time: 0:00
Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).
This bug belongs to the scope and closure bugs pattern
Every callback reports the last value, a counter never increments, UnboundLocalError on a variable you can see.
What went wrong: Array.prototype.fill() copies the same row array reference into every row. Modifying grid[r][c] mutates the shared row, updating every row simultaneously.
The fix: Use Array.from() with a mapping function to instantiate independent row arrays: Array.from({ length: rows }, () => new Array(cols).fill(0))
Remember: Never pass objects or arrays to .fill(); it populates the array with identical references.
The pattern behind it: scope and closure bugs.
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<iframe src="https://trybughunt.com/embed/e46d63b3-54ab-4a70-86ea-1e910b9b1978" title="Debugging challenge: Creating grid with array fill shares same row object reference" width="100%" height="580" loading="lazy" style="border:1px solid #d4d4d8;border-radius:12px;max-width:760px"></iframe>
<p style="font-size:14px">Debugging challenge from <a href="https://trybughunt.com/challenges/e46d63b3-54ab-4a70-86ea-1e910b9b1978">TryBugHunt</a></p>