Controlling Side Effects in React: How to Stop useEffect on Initial Render

In React, the useEffect hook is a powerful tool for performing side effects within functional components. Side effects can include fetching data, subscriptions, or any logic that interacts with the outside world. However, by default, useEffect runs after the initial render and after every subsequent re-render.

Why Skip the Initial Render?

There are several reasons why you might want to prevent useEffect from running on the initial render:

• To avoid unnecessary side effects (like API calls) when the component first mounts.
• To optimize performance by only triggering the effect when a dependency changes.
• To prevent unexpected behavior if your effect relies on state or props that haven't been initialized yet.

Approaches to Skip Initial Render

There are two common approaches to achieve this:

1. Using a useRef for Tracking Render State:

   • Create a useRef hook to store a flag indicating whether the component has been rendered for the first time.
   • In your useEffect callback, check the flag's value. If it's true (initial render), skip the side effect logic.
   • After the initial render (within a separate useEffect with an empty dependency array []), set the flag to false. This ensures the effect runs only on subsequent re-renders.

   import { useEffect, useRef } from 'react';
   
   function MyComponent() {
     const hasRendered = useRef(false);
   
     useEffect(() => {
       if (!hasRendered.current) {
         // Side effect logic here (won't run on initial render)
         // ...
       }
     }, [hasRendered]); // Only run when hasRendered changes
   
     useEffect(() => {
       hasRendered.current = true; // Set flag to false after first render
     }, []);
   
     return (
       // JSX content
     );
   }
   

2. Conditional Logic Based on State or Props:

   • If your side effect logic depends on a specific state value or prop being initialized, you can conditionally execute the effect within the callback based on that value.
   • This approach is simpler when the dependency is directly related to the effect behavior.

   import { useEffect, useState } from 'react';
   
   function MyComponent(props) {
     const [data, setData] = useState(null);
   
     useEffect(() => {
       if (data) {
         // Side effect logic here (only runs when data is available)
         // ...
       }
     }, [data]);
   
     // ... (fetch data or update data as needed)
   
     return (
       // JSX content
     );
   }
   

Choosing the Right Approach

• If your side effect is completely independent of component state or props, and you simply want to avoid it on the initial render, the useRef approach offers a more generic solution.
• If the effect's execution depends on the availability of specific state or props, using conditional logic might be more readable.

import { useEffect, useRef } from 'react';

function MyComponent() {
  const hasRendered = useRef(false);
  const [data, setData] = useState(null); // Example state for conditional rendering

  useEffect(() => {
    if (!hasRendered.current) {
      // Side effect logic here (won't run on initial render)
      console.log('Fetching data (only on subsequent renders)');
      fetch('https://api.example.com/data')
        .then(response => response.json())
        .then(fetchedData => setData(fetchedData));
    }
  }, [hasRendered]);

  useEffect(() => {
    hasRendered.current = true; // Set flag to false after first render
  }, []);

  return (
    <div>
      {data ? (
        <p>Fetched data: {JSON.stringify(data)}</p>
      ) : (
        <p>Loading data...</p>
      )}
    </div>
  );
}

Conditional Logic Approach:

import { useEffect, useState } from 'react';

function MyComponent(props) {
  const [data, setData] = useState(null);

  useEffect(() => {
    if (data) {
      // Side effect logic here (only runs when data is available)
      console.log('Processing data:', data);
      // ... your data processing logic
    }
  }, [data]);

  useEffect(() => {
    // Fetch data or update data as needed (example using props)
    if (!data && props.fetchData) {
      fetch('https://api.example.com/data')
        .then(response => response.json())
        .then(fetchedData => setData(fetchedData));
    }
  }, [data, props.fetchData]); // Only run when data or fetchData prop changes

  return (
    <div>
      {data ? (
        <p>Processed data: {JSON.stringify(data)}</p>
      ) : (
        <p>Loading data...</p>
      )}
    </div>
  );
}

• Instead of a separate useEffect to set the hasRendered flag to false after the first render, you can achieve the same effect by including hasRendered in the dependency array of the main useEffect that checks for it.
• When hasRendered changes from false to true on the first render, the effect will only run again on subsequent re-renders.

import { useEffect, useRef } from 'react';

function MyComponent() {
  const hasRendered = useRef(false);

  useEffect(() => {
    if (!hasRendered.current) {
      // Side effect logic here (won't run on initial render)
      console.log('Fetching data (only on subsequent renders)');
      // ...
    }
    hasRendered.current = true; // Update flag within the effect
  }, [hasRendered]);

  // ... (rest of your component)
}

Custom Hook for Reusability:

• Create a custom hook that encapsulates the logic of checking for the initial render and conditionally running the side effect.
• This promotes reusability across different components that need to control when useEffect runs.

import { useEffect, useRef } from 'react';

function useDidUpdateEffect(func) {
  const didMount = useRef(false);

  useEffect(() => {
    if (didMount.current) {
      func();
    } else {
      didMount.current = true;
    }
  }, [func]); // Re-run if func changes
}

function MyComponent() {
  const fetchData = () => {
    console.log('Fetching data');
    // ...
  };

  useDidUpdateEffect(fetchData);

  // ... (rest of your component)
}

• The basic useRef approach (approach 1) is often the most straightforward and widely used.
• The optimized useRef approach can be slightly more concise but might be less readable initially.
• A custom hook is beneficial if you need to reuse the logic across multiple components.
• Conditional logic based on state or props is suitable when the effect's execution depends directly on their availability.