Magic!, useEffect running twice 🤔

React's useEffect hook is an essential tool for managing side effects in your components. However, it's not uncommon to run into issues where the useEffect hook appears to be running twice in development mode, causing unexpected behavior in your application.

This issue occurs because React updates its components in a specific way during development mode. When a component re-renders, React first calls the component with the updated state and props, and then calls the useEffect hook with the same values. This ensures that the component and its effects are always in sync.

However, if you're not careful, this can lead to unexpected behavior. For example, consider the following code snippet:

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

const Example = () => {
  const [count, setCount] = useState(0);

  useEffect(() => {
    console.log('The count is:', count);
  }, [count]);

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

In this example, we have a simple component that displays a count and increments it when a button is clicked. The useEffect hook logs the current count to the console whenever it changes.

If you run this code in development mode, you'll notice that the useEffect hook is called twice for each update: once with the old count and once with the new count. This is because React is first calling the component with the old values, and then calling the component and its effects with the updated values.

To avoid this issue, it's important to understand how React updates its components in development mode. You can also use the useDebugValue hook to display a custom label for your hook in the React DevTools, making it easier to distinguish between the initial render and subsequent re-renders.

Here's an updated example that uses useDebugValue:

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

const Example = () => {
  const [count, setCount] = useState(0);

  useDebugValue(count, count => `Count: ${count}`);

  useEffect(() => {
    console.log('The count is:', count);
  }, [count]);

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

With this change, you can see the custom label in the React DevTools and easily distinguish between the initial render and subsequent re-renders.

In conclusion, understanding why the useEffect hook may run twice in development mode is essential for avoiding unexpected behavior in your application. By using the useDebugValue hook, you can easily identify and debug issues related to double rendering in your application.