Understanding useEffect Behavior in React: The Double Run with Empty Dependencies

• In React functional components, useEffect is a hook that allows you to perform side effects, such as data fetching, subscriptions, or manual DOM manipulation.
• It takes two arguments:
  • A callback function that contains the side effect logic.
  • (Optional) A dependency array that controls when the effect runs.

Expected Behavior with Empty Dependency Array:

• Intuitively, you might expect useEffect with an empty dependency array ([]) to run only once after the initial render, since there are no dependencies to track for changes.

Reasons for Running Twice (Development vs. Production):

• React Strict Mode (Development Only):

• React Reconciliation (Both Development and Production):

Mitigating Strategies (Generally Not Necessary):

• Conditional Logic (Consider with Caution):

• useRef Hook (Advanced):

General Guidance:

• In most cases, the double run of useEffect with an empty array won't have a noticeable performance impact. If it's a concern, consider the strategies above, but prioritize code clarity and maintainability.

Key Takeaways:

• Empty dependency arrays in useEffect are meant for effects that only need to run once after the initial render.
• Double execution might occur in development due to Strict Mode or in both development and production for internal reconciliation reasons.
• Mitigating strategies exist but should be used cautiously, as code clarity is often more important.

Example Codes:

import React, { useEffect } from 'react';

function MyComponent() {
  const [count, setCount] = React.useState(0); // State for demonstration

  useEffect(() => {
    console.log('useEffect called'); // This will log twice in development with Strict Mode
  }, []); // Empty dependency array

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

export default MyComponent;

import React, { useEffect } from 'react';

function MyComponent() {
  const [showDetails, setShowDetails] = React.useState(false); // State for demonstration

  useEffect(() => {
    console.log('useEffect called (potentially twice)');
    // Simulate side effect (e.g., data fetching)
    fetch('https://api.example.com/data')
      .then(response => response.json())
      .then(data => console.log('Fetched data:', data));
  }, [showDetails]); // Now depends on showDetails

  return (
    <div>
      <p>Show Details: {showDetails.toString()}</p>
      <button onClick={() => setShowDetails(!showDetails)}>Toggle Details</button>
    </div>
  );
}

export default MyComponent;

Explanation:

• The first example demonstrates how Strict Mode might cause the useEffect with an empty array to run twice in development. This is for testing purposes and doesn't necessarily happen in production.
• The second example shows a more typical scenario where useEffect runs with a specific dependency (showDetails). However, due to React's reconciliation process, it might run an extra time in some cases (depending on the complexity of the component and virtual DOM updates).

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

function MyComponent() {
  const hasRun = useRef(false); // Flag to track execution

  useEffect(() => {
    if (!hasRun.current) {
      console.log('useEffect called (once)');
      // Your side effect logic here
      hasRun.current = true;
    }
  }, []); // Empty dependency array

  return (
    <div>
      {/* Your component content */}
    </div>
  );
}

export default MyComponent;

• We introduce a useRef hook to create a flag (hasRun) that initially starts as false.
• Inside the useEffect callback, we check the value of hasRun.current. If it's false, it means the effect hasn't run yet, so we execute the side effect logic and update hasRun.current to true to prevent further execution.

Caution:

• This approach can make the code less readable and might introduce bugs if not careful. It's a good idea to use clear variable names and comments to explain the logic.
• Consider if the potential performance gain from preventing a single extra run outweighs the added complexity.

Custom Hook for Once-Only Effects (Advanced):

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

function useOnceEffect(callback) {
  const [hasRun, setHasRun] = useState(false);

  useEffect(() => {
    if (!hasRun) {
      callback();
      setHasRun(true);
    }
  }, [hasRun, callback]); // Depend on both hasRun and callback

  return null; // Custom hook doesn't return anything
}

function MyComponent() {
  useOnceEffect(() => {
    console.log('useEffect called (once)');
    // Your side effect logic here
  });

  return (
    <div>
      {/* Your component content */}
    </div>
  );
}

export default MyComponent;

• We create a custom hook called useOnceEffect that takes the side effect callback as an argument.
• The hook manages a state variable (hasRun) to track execution.
• Inside the useEffect hook within the custom hook, we check hasRun. If it's false, we execute the callback and update hasRun to true.
• The custom hook itself doesn't return anything.

• This approach involves creating and using a custom hook, which might be overkill for simple cases.
• As with conditional logic, consider if the complexity outweighs the benefit.