Deep Set II

Requirements

• Export deepSet(root, path, value) as named and default. root is a mutable plain object (including null-prototype objects) or array. path is a nonempty readonly array of nonempty strings or numeric array indexes. Return the same root object.
• Create missing intermediate containers. A following numeric segment creates an array; a following string segment creates a plain object. Replace scalar or non-plain intermediates when necessary. Traverse only own properties, never inherited ones.
• At the leaf, recursively merge two plain objects, concatenate two arrays in existing-then-incoming order, and otherwise replace with the incoming value. Nested merge conflicts use the same rules. An incoming undefined assigns an own property rather than deleting it.
• Plain-object and array containers installed at the leaf are cloned, including untouched parts of an existing merged leaf, so the incoming value is not aliased or mutated and the old leaf container remains untouched. Non-plain objects such as Date are atomic and may be shared.
• Mutate the path’s existing parent containers in place and preserve unrelated branches by identity. When merging two objects preserve the existing prototype; when cloning replacements preserve their source prototype. Arrays preserve holes and length; extra array properties, symbol/non-enumerable object properties, getters, and cycles are outside scope.
• Numeric path segments must be integers from 0 through 4294967294. Array containers require numeric segments. Reject __proto__, constructor, and prototype path segments, invalid segments, invalid roots, empty paths, and string access into arrays with TypeError before making any writes. Those names may exist as safe data keys inside leaf values.
• Use path arrays only, not dot-string parsing. Implement the behavior without a deep-set library. Input roots are mutable and extensible with ordinary writable data properties.

Examples

Input: const root = { user: { prefs: { theme: "light" } } }; deepSet(root, ["user", "prefs"], { compact: true });
Output: Returns root; root.user.prefs becomes { theme: "light", compact: true }.

Input: const root = { items: [1] }; deepSet(root, ["items"], [2, 3]);
Output: Returns root; root.items becomes [1, 2, 3].