Implement useDeferredValue in React

React's useDeferredValue hook is a powerful tool for optimizing performance by deferring the update of a non-critical part of the UI. This challenge asks you to recreate the functionality of useDeferredValue from scratch using fundamental React concepts. Understanding and implementing this hook will deepen your knowledge of React's reconciliation process and state management.

Problem Description

Your task is to create a custom React hook named useDeferredValue that mimics the behavior of React's built-in useDeferredValue. This hook will take a value as an argument and return a "deferred" version of that value. When the original value changes, the deferred value will not update immediately. Instead, it will wait for the current rendering to complete and for the browser to be idle before updating. This allows less important UI elements that depend on the deferred value to update without blocking the main thread or causing jank.

Key Requirements:

1. Hook Signature: The hook should have the signature useDeferredValue<T>(value: T): T.
2. Deferred Updates: When the input value changes, the hook should not immediately return the new value. Instead, it should continue to return the previous value for at least one render cycle, and then update to the new value when the browser is idle.
3. Immediate Update for Critical Renders: If the current render is considered "critical" (e.g., due to user interaction that might require immediate feedback), the deferred value should update synchronously with the original value. You'll need to simulate or infer this. For simplicity in this challenge, we'll consider any render after the initial render that's triggered by a change in the value to be potentially deferrable.
4. No External Libraries: You should use only core React features (useState, useEffect, useRef, useSyncExternalStore if necessary for advanced scenarios, though useState/useEffect should suffice for the core logic).

Expected Behavior:

• When the component mounts, the deferred value should be the initial value.
• When the original value changes, the deferred value should remain the previous value for one render cycle.
• In the subsequent render cycle, if the browser is idle, the deferred value should update to the new value.
• If the original value changes rapidly, the deferred value should lag behind, updating only after the user has stopped typing or a brief pause occurs.

Edge Cases:

• Initial Render: The deferred value should be the same as the initial value.
• Rapid Updates: The deferred value should exhibit a noticeable lag when the input value changes very quickly.
• Unmounting: Ensure no stale state updates occur after the component unmounts.

Examples

Example 1: Basic Text Input

Consider a component with a text input and a display of the input's value. We want to defer the update of the displayed value to keep the input responsive.

function App() {
  const [text, setText] = useState('');
  // Imagine this is where your custom useDeferredValue would be used
  // const deferredText = useDeferredValue(text);
  const deferredText = text; // For demonstration without the custom hook

  const handleChange = (e) => {
    setText(e.target.value);
  };

  return (
    <div>
      <input type="text" value={text} onChange={handleChange} />
      <p>Original Text: {text}</p>
      {/* This paragraph's update would be deferred */}
      <p>Deferred Text: {deferredText}</p>
    </div>
  );
}

Scenario:

1. User types "H".
2. text becomes "H".
3. The component re-renders. Without useDeferredValue, deferredText would also immediately become "H".
4. With useDeferredValue, deferredText would remain its previous value (e.g., "") for this render. A background task would schedule an update.
5. In the next idle render, deferredText updates to "H".
6. User types "e". text becomes "He".
7. deferredText remains "H" for this render.
8. In the next idle render, deferredText updates to "He".

Example 2: Expensive List Rendering

Imagine a component that renders a long list, and the items in the list are generated based on a filter value. We want to defer the update of the list so that the filter input remains responsive.

function MyListComponent({ filter }) {
  // Simulate an expensive computation or rendering of many items
  const items = Array.from({ length: 1000 }, (_, i) => `Item ${filter}-${i}`);

  return (
    <ul>
      {items.map((item, index) => (
        <li key={index}>{item}</li>
      ))}
    </ul>
  );
}

function App() {
  const [filter, setFilter] = useState('');
  // Imagine this is where your custom useDeferredValue would be used
  // const deferredFilter = useDeferredValue(filter);
  const deferredFilter = filter; // For demonstration

  const handleFilterChange = (e) => {
    setFilter(e.target.value);
  };

  return (
    <div>
      <input type="text" value={filter} onChange={handleFilterChange} placeholder="Filter items..." />
      {/* The MyListComponent would be passed the deferredFilter */}
      <MyListComponent filter={deferredFilter} />
    </div>
  );
}

Scenario:

1. User types "A" into the filter input.
2. filter becomes "A".
3. The component re-renders. If MyListComponent directly used filter, it would immediately try to re-render with "A". This could be slow.
4. With useDeferredValue, deferredFilter would remain its previous value (e.g., "") for this render cycle. The input remains responsive.
5. When the browser is idle, deferredFilter updates to "A", causing MyListComponent to re-render with the new filter.
6. User types "B". filter becomes "AB".
7. deferredFilter remains "A" for this render.
8. In the next idle render, deferredFilter updates to "AB".

Constraints

• Your custom useDeferredValue hook must be implemented in TypeScript.
• The hook should not rely on any internal React APIs that are not publicly exposed or are subject to change. Focus on core hooks like useState, useEffect, useRef.
• Performance: While you are not expected to perfectly match React's internal optimizations, the deferred value should noticeably lag behind the original value during rapid updates.
• No console logs or side effects within the hook itself, other than those necessary for its internal state management and scheduling.

Notes

• Scheduling: The core of this challenge lies in how you schedule the update of the deferred value. Consider using requestAnimationFrame or setTimeout with a delay, combined with a mechanism to track whether an update is pending and to cancel previous pending updates.
• "Critical" Renders: In a real-world scenario, React uses internal heuristics to determine if a render is "critical." For this challenge, you can simplify this: if a render is triggered by something other than a change in the deferred value itself, you can assume it might be a good candidate for deferral. However, the initial value and any value updates that happen after a long pause should update relatively quickly. A simple approach might be to always defer unless it's the very first render.
• useRef: You'll likely need useRef to store the latest value and manage pending updates without causing re-renders of the hook itself.
• useEffect: useEffect will be crucial for scheduling and cancelling updates after the component has rendered.
• useState: You'll need useState within your hook to hold the deferred value that is returned to the component.
• Reconciliation: Remember that React will re-render components that use your hook whenever the deferred value changes. Your hook's job is to control when that deferred value changes.

Success is defined by creating a useDeferredValue hook that accurately simulates the behavior of React's built-in hook, providing a noticeable lag between the original value and the deferred value during rapid updates, and ensuring the input remains responsive.