Immutability Utilities

Requirements

• Export update(value, spec) as named and default. spec is a plain object. A node is either an empty spec, nested child specs, or exactly one command with no other keys. Empty specs return the original value.
• Support $set for replacement (including undefined), $apply for a pure callback, and $merge for shallow own-enumerable field assignment between plain objects. $set/$apply return the replacement unchanged; $merge creates a new object only when an own field differs by Object.is or is newly added.
• Support $push and $unshift with array payloads, and $splice with an array of [start, deleteCount, ...items] instructions applied sequentially. start and deleteCount must be integers; deleteCount is nonnegative. Use normal splice clamping and negative-start behavior. Return the original array if final length and every element are Object.is-equal.
• Nested keys update own fields on plain objects or existing array indexes. Missing object fields may be created through a $set/$apply child; intermediate containers are not auto-created. Array child keys must be canonical nonnegative integer strings below length. Clone only changed ancestors and preserve other branch references.
• Throw TypeError for invalid specs, unknown commands, mixed command/child keys, wrong command target/payload types, invalid splice instructions, and __proto__/constructor/prototype nested traversal keys. $merge may contain those names as safely defined data keys.
• Do not mutate value, specs, command arrays, or inserted values. Inserted containers may be shared by reference. Preserve plain-object/null-object prototypes. Inputs use acyclic ordinary data and dense arrays; getters, symbols, non-enumerable fields, deep merge, and impure callbacks are outside scope. TypeScript returns unknown because updates may change types.

Examples

Input: update({ user: { score: 1 }, tags: ["a"] }, { user: { score: { $apply: n => n + 1 } }, tags: { $push: ["b"] } });
Output: { user: { score: 2 }, tags: ["a", "b"] }

Input: const original = { x: 1 }; update(original, { x: { $set: 1 } }) === original;
Output: true