React Hook: useFirstMountState

This challenge asks you to create a custom React hook, useFirstMountState, that tracks whether a component has been mounted for the first time. This is a common utility in React development, particularly useful for implementing logic that should only run once on initial render, such as data fetching or setting up event listeners, without relying on potentially complex useEffect dependency arrays or manual boolean flags.

Problem Description

You need to implement a TypeScript hook named useFirstMountState. This hook should return a boolean value indicating whether the component using the hook is currently on its first mount.

Key Requirements:

• The hook should return true on the initial render of the component and false on all subsequent re-renders.
• The hook should be implemented using standard React hooks (useState, useRef).
• The hook must be written in TypeScript and correctly typed.

Expected Behavior:

A component that uses useFirstMountState will receive a boolean value. This value will be true only during the very first render cycle of that component. After the first render, the value returned by the hook will consistently be false.

Edge Cases to Consider:

• Strict Mode: How does the hook behave when React's Strict Mode is enabled, which intentionally re-renders components twice in development? The hook should still correctly identify the actual first mount.
• Unmounting and Re-mounting: If a component unmounts and then remounts, the hook should correctly report true for the new mount.

Examples

Example 1:

import React from 'react';
import { useFirstMountState } from './useFirstMountState'; // Assuming your hook is in this file

function MyComponent() {
  const isFirstMount = useFirstMountState();

  React.useEffect(() => {
    console.log('Effect running. Is first mount?', isFirstMount);
    // This effect's logic should ideally run only once on the first mount.
    // The `isFirstMount` value here will reflect the state *during* render.
  }, []); // Empty dependency array for effect

  return (
    <div>
      <p>Is this the first mount? {isFirstMount ? 'Yes' : 'No'}</p>
    </div>
  );
}

// In App.js (or similar)
function App() {
  const [show, setShow] = React.useState(true);

  return (
    <div>
      <button onClick={() => setShow(!show)}>Toggle Component</button>
      {show && <MyComponent />}
    </div>
  );
}

Input: The App component renders MyComponent for the first time. Output (during first render of MyComponent):

• The p tag displays: "Is this the first mount? Yes"
• The console.log inside useEffect might output: "Effect running. Is first mount? true" (depending on when the effect runs relative to the render).

Explanation: On the very first render of MyComponent, useFirstMountState() returns true.

Example 2:

// Continuing from Example 1:
// User clicks the "Toggle Component" button to unmount and remount MyComponent.

Input: The App component toggles show to false, then true again, causing MyComponent to unmount and then remount. Output (during the second render of MyComponent):

• The p tag displays: "Is this the first mount? Yes"
• The console.log inside useEffect might output: "Effect running. Is first mount? true"

Explanation: Even though the component has rendered before, after unmounting and re-mounting, useFirstMountState() correctly identifies this as the first mount of this new instance and returns true.

Example 3:

// Component that re-renders due to parent state change, but isn't unmounted.
import React from 'react';
import { useFirstMountState } from './useFirstMountState';

function AnotherComponent() {
  const isFirstMount = useFirstMountState();
  const [count, setCount] = React.useState(0);

  console.log('Render. Is first mount?', isFirstMount);

  return (
    <div>
      <p>Is this the first mount? {isFirstMount ? 'Yes' : 'No'}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

// In App.js (or similar)
function App() {
  const [show, setShow] = React.useState(true);

  return (
    <div>
      <button onClick={() => setShow(!show)}>Toggle Component</button>
      {show && <AnotherComponent />}
    </div>
  );
}

Input: AnotherComponent is rendered for the first time. The user then clicks the "Increment" button within AnotherComponent, causing it to re-render. Output (during first render of AnotherComponent):

• The p tag displays: "Is this the first mount? Yes"
• The console.log displays: "Render. Is first mount? true"

Output (during subsequent render of AnotherComponent after clicking "Increment"):

• The p tag displays: "Is this the first mount? No"
• The console.log displays: "Render. Is first mount? false"

Explanation: The hook correctly returns true only on the initial render. On subsequent re-renders triggered by internal state changes (or parent prop changes), it returns false.

Constraints

• The hook must be implemented using only built-in React hooks.
• The solution must be written in TypeScript.
• The hook should have minimal performance impact. It should not introduce significant render-blocking delays or excessive memory usage.

Notes

Consider how you can use useRef to persist a value across renders without causing re-renders itself. This will be key to tracking the "mounted" state.

Think about the lifecycle of a React component and when you can reliably determine if it's the first time it's being rendered to the DOM.

When considering Strict Mode, remember that it might call render twice for components in development. Your hook's logic should correctly identify the actual initial mount, not just the first render pass within Strict Mode.