JavaScript Arrays Mastery Quiz

Review

1. What is the key difference between a hole and an explicit undefined element?

   A hole is absent at that index. A read still yields undefined, but property existence and some array-method behavior can distinguish it.
2. Which operation changes the original array?

   splice edits the receiver. The other listed operations create or return results without directly changing the receiver.
3. What does [...items] copy?

   Array spread creates a new outer array while preserving the element values; nested objects are not recursively cloned.
4. Which method is intended to produce one transformed result per visited element?

   map constructs a new array from callback results, preserving the visited element count and order.
5. What is the optional second argument to reduce?

   The second argument supplies the accumulator’s initial value and makes empty-input cases predictable when an identity value is appropriate.
6. Which equality algorithm does includes use?

   Array.prototype.includes compares with SameValueZero, so for example NaN can be found by includes.
7. Why can [10, 2, 30].sort() surprise you?

   Without a comparator, Array.prototype.sort compares elements according to string conversion/order rather than numeric magnitude.
8. Which method replaces one array position without mutating the receiver?

   with returns a new array with the selected position replaced and leaves the original array unchanged.
9. What does Array.fromAsync return?

   Array.fromAsync is asynchronous and returns a Promise for the resulting array.
10. What is a useful distinction between Object.groupBy and Map.groupBy?

    Object.groupBy converts callback results to property keys; Map.groupBy keeps them as Map keys.