Implement useUpdateEffect Hook in React

Your challenge is to create a custom React hook, useUpdateEffect, that behaves similarly to useEffect but only runs its effect after the component has mounted and subsequent re-renders. This is useful for scenarios where you need to perform an action based on changes in dependencies but want to avoid the initial run on mount.

Problem Description

You need to implement a hook called useUpdateEffect that accepts two arguments:

1. effect: A function that contains the side effect logic. This function can optionally return a cleanup function.
2. dependencies: An array of dependencies. The effect will re-run if any of these dependencies change.

The key requirement is that the effect function should not run on the initial mount of the component. It should only execute on subsequent re-renders where at least one of the dependencies has changed. If no dependencies are provided, the effect should still not run on mount but should run on every subsequent re-render.

Key Requirements:

• The effect function should not execute on the initial render of the component.
• The effect function should execute on subsequent renders if any dependency in the dependencies array has changed.
• The hook should correctly handle cleanup functions returned by the effect.
• The hook should mimic the behavior of useEffect in all other aspects, including dependency array handling (empty array, array with dependencies, no array).

Expected Behavior:

• On initial mount: The effect function is skipped.
• On subsequent re-renders:
  • If dependencies is provided and a dependency has changed, the effect runs, and any previous cleanup is executed.
  • If dependencies is provided and no dependencies have changed, the effect is skipped.
  • If dependencies is not provided (i.e., undefined), the effect runs on every re-render after the initial mount.

Edge Cases:

• No dependencies provided: The effect should run on every re-render after the initial mount.
• Empty dependencies array ([]): The effect should only run on subsequent re-renders if the component re-renders for reasons other than the initial mount (effectively, it won't run on mount, but it also won't run on subsequent renders if the dependencies haven't changed, which in this case, they never will. This is similar to useEffect with [] but without the initial run).
• Dependencies changing: Ensure correct comparison of dependencies to trigger the effect.

Examples

Example 1: Basic Usage with Dependencies

Consider a component that fetches data when a userId changes.

import React, { useState, useEffect } from 'react';

// Assume useUpdateEffect is implemented correctly
// import { useUpdateEffect } from './useUpdateEffect';

function UserProfile({ userId }: { userId: number }) {
  const [userData, setUserData] = useState(null);
  const [loading, setLoading] = useState(false);

  // Placeholder for actual data fetching logic
  const fetchUserData = async (id: number) => {
    console.log(`Fetching data for user ID: ${id}`);
    setLoading(true);
    // Simulate API call
    await new Promise(resolve => setTimeout(resolve, 500));
    setUserData({ id: id, name: `User ${id}` });
    setLoading(false);
  };

  // This effect should NOT run on initial mount, only when userId changes
  useUpdateEffect(() => {
    fetchUserData(userId);
    return () => {
      console.log(`Cleaning up for user ID: ${userId}`);
      // Potentially cancel ongoing fetch requests
    };
  }, [userId]);

  return (
    <div>
      <h2>User Profile</h2>
      {loading ? (
        <p>Loading...</p>
      ) : userData ? (
        <p>Name: {userData.name}</p>
      ) : (
        <p>No user data available.</p>
      )}
    </div>
  );
}

function App() {
  const [id, setId] = useState(1);

  console.log("App rendering...");

  return (
    <div>
      <button onClick={() => setId(id + 1)}>Next User</button>
      <button onClick={() => setId(1)}>Reset to User 1</button>
      <UserProfile userId={id} />
    </div>
  );
}

export default App;

Expected Console Output when running App and clicking "Next User" multiple times:

App rendering...
Fetching data for user ID: 1 // This won't happen if useUpdateEffect is correctly implemented
App rendering...
Fetching data for user ID: 2
App rendering...
Cleaning up for user ID: 2
Fetching data for user ID: 3
App rendering...
Cleaning up for user ID: 3
Fetching data for user ID: 4

Wait, the example is a bit misleading. Let's correct the expected behavior for useUpdateEffect:

Corrected Expected Console Output when running App and clicking "Next User" multiple times:

App rendering...
App rendering...
Fetching data for user ID: 2
App rendering...
Cleaning up for user ID: 2
Fetching data for user ID: 3
App rendering...
Cleaning up for user ID: 3
Fetching data for user ID: 4

Explanation:

1. On the initial render of App, UserProfile mounts with userId: 1. useUpdateEffect does not run.
2. When "Next User" is clicked, App re-renders with userId: 2. useUpdateEffect detects the change in userId, so it runs fetchUserData(2).
3. When "Next User" is clicked again, App re-renders with userId: 3. The previous effect for userId: 2 is cleaned up, and fetchUserData(3) runs.

Example 2: No Dependencies (Runs on every update after mount)

import React, { useState, useEffect } from 'react';

// Assume useUpdateEffect is implemented correctly
// import { useUpdateEffect } from './useUpdateEffect';

function Counter() {
  const [count, setCount] = useState(0);

  console.log("Counter rendering...");

  // This effect should NOT run on initial mount, but on every subsequent re-render
  useUpdateEffect(() => {
    console.log(`Counter updated to: ${count}`);
    // Note: This would run on every re-render AFTER the first one.
    // If you truly want it to run *every* time the component updates
    // and you don't care about cleanup, this is how you'd do it.
    // For a more robust use case, you'd typically have dependencies.
  }); // No dependency array means it runs on all updates after mount

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

export default Counter;

Expected Console Output:

Counter rendering...
Counter rendering...
Counter updated to: 1
Counter rendering...
Counter updated to: 2
Counter rendering...
Counter updated to: 3

Explanation:

1. On initial mount, Counter renders. useUpdateEffect does not run.
2. When "Increment" is clicked, Counter re-renders. useUpdateEffect runs and logs the new count. This happens for every subsequent increment.

Example 3: Empty Dependencies Array ([])

import React, { useState, useEffect } from 'react';

// Assume useUpdateEffect is implemented correctly
// import { useUpdateEffect } from './useUpdateEffect';

function LifeCycleLogger() {
  const [renderCount, setRenderCount] = useState(0);

  console.log("LifeCycleLogger rendering...");

  // This effect should only run if the component re-renders, but NOT on the initial mount.
  // With an empty dependency array, useEffect would run only on mount.
  // useUpdateEffect with [] should prevent the initial mount run.
  useUpdateEffect(() => {
    console.log("LifeCycleLogger: Effect ran after mount.");
    return () => {
      console.log("LifeCycleLogger: Cleanup after effect.");
    };
  }, []); // Empty dependency array

  return (
    <div>
      <p>Render Count: {renderCount}</p>
      <button onClick={() => setRenderCount(renderCount + 1)}>Trigger Re-render</button>
    </div>
  );
}

export default LifeCycleLogger;

Expected Console Output:

LifeCycleLogger rendering...
LifeCycleLogger rendering...
LifeCycleLogger: Effect ran after mount.
LifeCycleLogger rendering...
LifeCycleLogger: Cleanup after effect.
LifeCycleLogger: Effect ran after mount.
LifeCycleLogger rendering...
LifeCycleLogger: Cleanup after effect.
LifeCycleLogger: Effect ran after mount.

Explanation:

1. On initial mount, LifeCycleLogger renders. useUpdateEffect with [] does not run.
2. When "Trigger Re-render" is clicked, the component re-renders. useUpdateEffect detects the re-render (even though dependencies are []), cleans up the previous (non-existent on first run) effect, and runs the new effect.

Constraints

• The implementation must be in TypeScript.
• The hook should correctly handle the comparison of primitive types and object/array references for dependencies.
• Performance is important; avoid unnecessary re-renders or computations.

Notes

• Consider how useEffect works and how you can adapt its behavior.
• You will likely need to use useRef to keep track of whether the component has mounted and to store the previous dependencies for comparison.
• Think carefully about the timing of your effect execution and cleanup.
• The built-in useEffect hook can be used internally to achieve the desired behavior.