every() and the other array callbacks skip the holes of new Array(n), so a fresh sparse array passes every test

Fact

new Array(n) makes n holes, not n undefineds, and every, some, forEach, filter, map and reduce never call their callback for a hole. So new Array(3).every((x) => x !== undefined) is true, and some on it is false. includes, indexOf's cousin findIndex, find, spreading and for…of do visit holes, as undefined.

Why it matters

The natural way to collect numbered pieces as they arrive is to allocate new Array(total) and fill slots. A "have all pieces arrived?" check written with every(p => p !== undefined) then reports a buffer with one piece in it as complete, and code that should discard or wait on a partial transfer treats it as whole. Nothing throws; the test just answers wrong.

How to apply

• Ask about missing slots with parts.includes(undefined), which treats holes as undefined, or keep an explicit count of filled slots.
• Or allocate with Array.from({ length: n }) / new Array(n).fill(undefined) when callbacks must see every index.
• In tests, cover the partly-filled case: a buffer with only slot 0 set.