Why useEffect Runs Twice in React and How to Fix It

The Role of useEffect in Managing Side Effects
- What is useEffect?
- When Should You Use It?
- Basic useEffect Example
The Mystery of useEffect Running Twice
- Common Misconceptions About useEffect
- Why React Isn’t Broken
Why is useEffect Executing Twice?
- Strict Mode: A Double-Edged Sword
- Development Mode vs. Production Mode
Diagnosing useEffect Running Twice
- Identifying the Real Causes
- Analyzing Lifecycle and Mounting Patterns
How to Prevent useEffect from Running Twice
- Disabling Strict Mode: Pros and Cons
- Optimizing Dependencies the Right Way
Building a Custom Hook to Manage useEffect Execution Efficiently
- Reusable useRunOnceEffect Hook
- Applying the Hook in Real Projects
Essential useEffect Practices to Prevent Repeated Executions
- Managing Dependency Arrays Like a Pro
- Using Cleanup Functions to Avoid Memory Leaks
Common Pitfalls and How to Solve Them
- The Infinite Loop Dilemma
- Preventing Unwanted Re-renders and Redundant API Calls
Advanced Techniques and React 18 Updates
- Replacing useEffect with React Query or SWR
- React 18 Features That Affect useEffect
- Automatic Batching and Transitions
Conclusion
- Final Thoughts on Handling useEffect
- Embracing React’s Debugging Tools for Cleaner Code

The Role of useEffect in Managing Side Effects

The useEffect hook is designed to run logic after a component renders. This includes:

Fetching data
Setting up subscriptions
Interacting with browser APIs (like localStorage or window)
Cleaning up on component unmount

useEffect(() => {
  console.log("Component mounted");
  return () => console.log("Component unmounted");
}, []);

It can be a powerful tool — when used correctly.

The Mystery of useEffect Running Twice

Common Misconceptions About useEffect Behavior

Many developers think React is broken when they see useEffect executing more than once — but it's not. It’s actually by design.

Strict Mode and Its Impact on useEffect

In React 18, Strict Mode was enhanced to intentionally double-invoke certain functions, including useEffect, to detect side effects.

Why is useEffect Executing Twice?

Strict Mode: A Double-Edged Sword

React Strict Mode in development re-mounts components once to:

Detect unexpected side effects
Help you write safer components

This is only in development mode, not in production.

<React.StrictMode>
  <App />
</React.StrictMode>

Development Mode Versus Production Mode

You won’t see useEffect run twice in a production build, which means users won’t be affected — it’s for you, the developer.

Diagnosing useEffect Running Twice

Identifying the Causes

Common causes for useEffect executing more than expected:

Using unstable functions or props in dependency array
Fetching data inside a render cycle
Not cleaning up properly

Analyzing Component Lifecycle and Mounting Behavior

Use logging or profiling tools to track renders and useEffect behavior over time.

How to Prevent useEffect from Running Twice

Disabling Strict Mode: Pros and Cons

Yes, you could remove <React.StrictMode> but:

You'll lose all the helpful development checks
Not a recommended long-term solution

Optimizing Dependencies for useEffect

Make sure only stable values are in the dependency array:

useEffect(() => {
  fetchData();
}, []); // Only once

Avoid functions or objects that are re-created every render.

Crafting a Custom Hook to Control useEffect Execution

Creating a Reusable Hook

Use a ref to ensure your effect runs only once:

function useRunOnceEffect(callback) {
  const hasRun = useRef(false);
  useEffect(() => {
    if (!hasRun.current) {
      callback();
      hasRun.current = true;
    }
  }, []);
}

Ensuring useEffect Runs Only Once

Use this in your components instead of default useEffect:

useRunOnceEffect(() => {
  // Your one-time logic
});

Best Practices for useEffect to Avoid Multiple Calls

Managing Dependencies Correctly

Only include variables that will truly change. Avoid inline functions or complex objects in dependencies.

Leveraging Cleanup Functions to Prevent Memory Leaks

Always clean up effects like event listeners or timers:

useEffect(() => {
  const timer = setInterval(() => doSomething(), 1000);
  return () => clearInterval(timer);
}, []);

Common Pitfalls and How to Solve Them

Debugging the useEffect Loop: How to Identify and Resolve the Core Issue

Watch for infinite loops caused by state updates within useEffect:

useEffect(() => {
  setCount(count + 1); // ❌ Infinite loop
}, [count]);

Instead, structure logic outside of reactive updates, or use useRef or useCallback.

Avoiding Unnecessary Rerenders and API Calls

Use libraries like React Query or SWR that handle caching and revalidation automatically.

Advanced Techniques and React 18 Updates

React Query for Data Fetching

Instead of using useEffect for every data fetch, try React Query:

const { data, error } = useQuery('todos', fetchTodos);

It reduces the need for manual useEffect usage and avoids double-fetching.

React's Latest Features and Their Impact on useEffect Execution

React 18 introduced automatic batching, concurrent rendering, and transitions — all of which can affect how and when effects are fired.

Keep your side effects clean, batched, and idempotent.

Conclusion

While useEffect running twice might seem like a bug, it's a helpful feature when used correctly. Embrace it as a debugging tool. Use custom hooks, cleanup functions, and libraries to manage effects more efficiently. With a better understanding of Strict Mode and React's lifecycle, you’ll write cleaner, bug-free components.

FAQs – Why useEffect Runs Twice

Q1. Why is useEffect running twice in React 18

Because React 18’s Strict Mode intentionally invokes effects twice in development to help identify bugs related to side effects that don’t clean up properly. This does not happen in production builds.

Q2. Does this double execution happen in production?

No. This behavior is exclusive to development mode with React Strict Mode enabled. In production mode, useEffect executes just once — as it should.

Q3. Can I disable Strict Mode to stop useEffect from running twice?

Yes, you can disable Strict Mode, but it’s not recommended unless you're debugging or testing. It’s there to help catch memory leaks and other subtle issues.

Q4. How do I ensure my effect runs only once?

Use an empty dependency array [] and make sure your code is free of side effects that persist without proper cleanup. Optionally, use a custom hook to guard first-time execution.

Q5. Why is my API being called twice in useEffect?

It’s likely due to Strict Mode’s double invocation in development or an unintentional rerender triggered by a state change or props update.

Q6. Can useEffect be called multiple times on purpose?

Yes! By specifying dependencies in the array, you can tell React to re-run the effect whenever those dependencies change. It’s by design, not a bug.

Q7. What’s the difference between useEffect in React 17 and 18?

React 18 introduces automatic batching and enhanced Strict Mode checks, which may cause effects to run twice for mount/unmount cycles in dev mode only.

Q8. How do I stop infinite loops in useEffect?

Check your dependency array and ensure you're not updating state that causes the component to rerender endlessly. Memoize functions or values if needed.

Q9. Is it safe to use multiple useEffect hooks in one component?

Yes! React allows you to separate concerns by using multiple useEffect hooks, each with its own dependencies.

Q10. Should I use useLayoutEffect instead to avoid this?

Not really. useLayoutEffect also runs twice in Strict Mode and blocks painting. Use it only when you need to synchronously read layout before the browser paints.