Immutable Array

Requirements

• Export ImmutableArray<T> as named and default. Its constructor accepts a readonly array and copies that array; omitting the argument creates an empty wrapper. Expose a read-only length getter, get(index), and toArray(). get uses nonnegative integer indexes and returns undefined for invalid/out-of-range indexes; toArray returns a fresh shallow copy.
• Implement set(index, value), push(...items), unshift(...items), pop(), shift(), and splice(start, deleteCount, ...items). Updates return wrappers and never mutate the original wrapper or its input array. Elements are shallowly shared by reference.
• set requires an existing nonnegative integer index; invalid indexes throw RangeError. If Object.is(existing, value), return the same wrapper. Otherwise return a new wrapper with that entry replaced.
• push/unshift with no items and pop/shift on empty arrays return the same wrapper. splice uses ordinary array clamping and negative-start semantics; start/deleteCount must be integers and deleteCount nonnegative, otherwise throw RangeError.
• After splice, return the same wrapper if length and every element are Object.is-equal to the original; otherwise return a new wrapper. Dense acyclic arrays are assumed. No deep cloning, mutation methods on elements, sparse-array support, iterator implementation, or complete Array API is required.
• Copies returned by toArray must not expose internal storage. TypeScript preserves the element type through all operations; set/push/unshift/splice items must be assignable to T.

Examples

Input: const a = new ImmutableArray([1, 2]); const b = a.push(3).set(0, 9);
a.toArray(); b.toArray();
Output: [1, 2] and [9, 2, 3]

Input: const a = new ImmutableArray([NaN]); a.set(0, NaN) === a;
Output: true