Time: 0:00
Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).
This bug belongs to the async and race conditions pattern
A missing await, results in the wrong order, and updates that overwrite each other under load.
What went wrong: the function starts a timer and returns immediately. The timer fills in result later, but by then the function has already returned undefined.
The fix: return the promise, so the caller can wait for it: return new Promise((resolve) => setTimeout(() => resolve(n * 2), 10));
Remember: a value that arrives later must be returned as a promise, not stored in a variable.
The pattern behind it: async and race conditions.
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