Why Does My useEffect Hook Run Twice in React 18 (and How Can I Fix It)?

Why useEffect Runs Twice and How to Handle It in React

Problem:

When using useEffect to log state changes on mount, it is observed that the effect gets triggered twice. This issue is encountered in React 18.

Reason:

In React 18's development mode with StrictMode, useEffect is intentionally remounted once to ensure that critical components are resilient to multiple mounting and unmounting cycles. React aims to preserve state during these cycles for better performance.

Solution:

Instead of relying on the initial run of useEffect, it is recommended to use an empty dependency array ([]) which runs the effect only after the first render:

import { useState, useEffect } from "react";

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

  useEffect(() => {
    console.log("rendered", count);
  }, []);

  return <div>...</div>;
};

Alternative Solutions:

If the dependency on the state is essential and must be checked on every render, consider using the following approaches:

• Cleanup Function: Use the cleanup function returned by useEffect to perform side effects when the component unmounts. This ensures proper cleanup and avoids memory leaks.
• useRef: The useRef hook can be used to store references to mutable data, and its value persists between renders. This can be useful for tracking state changes without triggering multiple useEffect calls.
• Caching and Debouncing: If the effect involves making external requests, consider using caching or debouncing techniques to prevent duplicate calls.

Production Behavior:

It is important to note that this behavior only occurs in development mode. In production builds, useEffect will run only once per component mount.

Conclusion:

Understanding why useEffect runs twice in React 18 is crucial for writing well-behaved and robust React code. By employing the appropriate solutions discussed above, developers can prevent bugs, maintain state consistency, and ensure efficient application performance.

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