Implementing Copy-on-Write State Management in React

This challenge focuses on building a fundamental state management pattern in React: Copy-on-Write. This pattern is crucial for ensuring immutability, which is vital for React's reconciliation algorithm and efficient state updates. You will create a custom hook that allows components to manage state where any modification results in a new copy of the state object, rather than direct mutation.

Problem Description

Your task is to implement a custom React hook, useCopyOnWriteState, that mimics the behavior of useState but enforces immutability through a copy-on-write strategy. This hook should return the current state value and a function to update it. However, the update function must not mutate the original state object. Instead, it should return a new state object with the requested changes.

Key Requirements:

1. useCopyOnWriteState Hook: Create a TypeScript hook named useCopyOnWriteState.
2. Return Values: The hook should return an array with two elements:
   • The current state value.
   • An update function.
3. Immutability: The update function must accept either a new state value or a function that receives the current state and returns the new state. Crucially, any modification to the state, whether by direct assignment of a new value or by returning a modified version of the previous state, must result in a completely new state object. The original state object should never be mutated.
4. Type Safety: Leverage TypeScript to ensure type safety for the state and the update function.

Expected Behavior:

When the update function is called, the hook should:

• If a new state value is provided, it should be used directly as the new state.
• If a function is provided, it should be called with the current state, and its return value should be the new state.
• In both cases, the new state object must be a distinct copy from the previous one.

Edge Cases:

• Initial state can be any valid type.
• The update function should handle cases where the provided update function might return the exact same object reference (though ideally, your implementation should still create a new object to guarantee the pattern).

Examples

Example 1:

// In a React Component:
const [count, setCount] = useCopyOnWriteState(0);

// User clicks a button, triggering:
setCount(prevCount => prevCount + 1);

// Internally:
// If initial state was `0`, and the update function is called,
// the new state will be `1`. The hook ensures a new state object/value is assigned.

Example 2:

// In a React Component:
interface User {
  name: string;
  address: {
    street: string;
    city: string;
  };
}

const initialUser: User = {
  name: "Alice",
  address: {
    street: "123 Main St",
    city: "Anytown",
  },
};

const [user, setUser] = useCopyOnWriteState<User>(initialUser);

// User updates their city, triggering:
setUser(prevUser => ({
  ...prevUser, // Creates a new user object
  address: {
    ...prevUser.address, // Creates a new address object
    city: "Newville",
  },
}));

// Internally:
// The `setUser` call creates a new `user` object and a new `address` object within it.
// The original `user` and `user.address` objects are not mutated.

Constraints

• The useCopyOnWriteState hook must be implemented using standard React hooks (useState, useCallback).
• The solution must be in TypeScript.
• The primary goal is to demonstrate the copy-on-write pattern for immutability. Deep copying complex nested objects within the update function is not the responsibility of the hook itself, but rather the responsibility of the caller using the update function (as shown in Example 2). The hook's responsibility is to ensure the top-level state is updated immutably.

Notes

• Think about how React's useState works and how you can intercept the update to ensure a new state reference is always set.
• Consider the difference between primitive types and object types in JavaScript and how immutability applies to each.
• The useCallback hook can be useful for memoizing the update function to prevent unnecessary re-renders of child components.
• The challenge is to ensure that even if the update function returns the exact same value reference (e.g., for primitives), the hook's internal state update mechanism treats it as a potential change and assigns it. However, the spirit of copy-on-write means the caller provides a new structure when modifying objects.