Bug

A List grown past 32 elements while every value is undefined reads those values back as null, corrupting get, iteration, toArray, equals and hashCode.

List().setSize(32).setSize(33).get(0); // => null   (expected: undefined)

This affects any path that produces an all-undefined region spanning the 32-element (tail → root) boundary — setSize, push(undefined), mutations, and the unshift/delete/shift sequence from #2230.

Root cause

A List represents an absent trie node as undefined almost everywhere (clear(), emptyList(), and the trie's own array holes via removeBefore). But _root was stored as null in two spots:

• the small-List constructor: makeList(0, size, SHIFT, null, …)
• the "origin within tail" branch of setListBounds: newRoot = null

For small lists this is invisible (indices route to the tail, never the root). But once such a List grows past the next 32-element boundary, in-range indices route through _root, and the read idiom node && node.array[i] returns the falsy node itself — null && x === null leaks null, whereas undefined && x === undefined reads correctly.

Fix

Normalize both sites to undefined so an absent root has a single sentinel, consistent with the rest of the file. No reader change is needed: node && node.array[i] is correct once _root is VNode | undefined. JSDoc is added on VNode / makeList / listNodeFor to capture that shape ahead of the TypeScript migration.

Notes

This supersedes #2230: that change guards the "Merge Tail into tree" step, but its condition starts with oldTail && …, so it does not cover the setSize(≤32).setSize(≥33) family (where oldTail is itself null) — setSize(32).setSize(33) still returns null with it applied.

Tests

8 regression cases in __tests__/List.ts (minimal repro + variants, the #2230 sequence, and a null-round-trips guard against over-correction). Full suite green (826 tests, incl. fast-check property tests).