How useEffect Works Behind the Scenes in React

You've probably used useEffect hundreds of times by now. Maybe you've even mastered the dependency array dance (we've all been there). But here's something that might surprise you. Have you ever stopped to wonder what actually happens when React sees that useEffect call? If you've read about how useState works under the hood, you'll recognize some familiar patterns in the fiber architecture that powers both hooks.

I'll be honest with you. It's way more fascinating than you might think. React doesn't just run your effect function whenever it feels like it. There's this entire coordination happening behind the scenes, complete with scheduling, cleanup methods, and some pretty clever optimizations that make everything work smoothly.

Today, we're going to peel back the curtain and see exactly how useEffect works under the hood. We'll explore React's Fiber architecture, understand the render and commit phases, and discover why your effects sometimes behave in ways that seem... well, almost magical.

The Foundation: React Fiber Architecture

Here's where things get really interesting. Every React component you write gets transformed into something called a fiber node. Think of it as React's internal blueprint for your component. This isn't just some abstract concept floating around; it's a real data structure that React uses to track everything about your component. This same fiber architecture is what powers useState's state management system, creating a unified foundation for all React hooks.

To understand this better, imagine you're building a house. The fiber node is like the architectural blueprint that contains every single detail where the electrical outlets go, how the plumbing connects, what materials are used, and how everything fits together. Just like a blueprint helps contractors understand the house's structure, the fiber node helps React understand your component's structure and state.

Now, here's where it gets cool. When you call useEffect, React doesn't immediately run your function. Instead, it creates what's called an Effect object and attaches it to your component's fiber. This object is like a detailed instruction manual that contains everything React needs to manage your effect's entire lifecycle.

Let me break down what's inside this Effect object and why each piece matters:

tag – This is React's way of marking whether your effect should actually run (based on whether your dependencies changed). Think of it like a flag on a mailbox that tells the mail carrier whether there's mail to deliver. React uses this tag to decide if your effect needs to execute.
create() – This is your actual effect callback function (the first argument you pass to useEffect). It's the function that contains your side effect logic. The code that fetches data, sets up subscriptions, or manipulates the DOM.
destroy() – This gets set to your cleanup function after your effect runs. Initially, this is null, but if your effect returns a cleanup function, React stores it here. It's like having a "cleanup crew" ready to tidy up after your effect finishes.
deps – Your dependency array, which React uses to figure out if anything actually changed. This is crucial for performance. React compares these dependencies to determine if your effect needs to run again.

It's like React is creating a detailed to-do list for your side effects, complete with instructions on when to run them and how to clean them up. But here's the key insight. This isn't just a simple list. It's an advanced tracking system that allows React to optimize when and how your effects run.

Render vs. Commit Phases: The Two-Step Dance

This is where many developers get confused, so let me walk you through it step by step. React's rendering process happens in two distinct phases, and understanding this is absolutely crucial for grasping how useEffect works.

The Render Phase: Planning What Needs to Change

During the render phase, React is basically asking "What needs to change?" It runs your component function, compares the virtual DOM, and figures out what updates are necessary. This is where React registers your useEffect calls. But here's the thing, it doesn't run them yet.

Think of it like planning a party. You're making the guest list, deciding what food to order, and figuring out the logistics. But you're not actually throwing the party yet. You're just preparing everything so that when the time comes, everything runs smoothly.

The render phase is also where React performs what's called "reconciliation". The process of comparing the new virtual DOM tree with the previous one to determine what actually needs to be updated. This takes a lot of computer power, which is why React is so careful about when it runs effects.

The Commit Phase: Making It Happen

After React figures out what needs to change, it enters the commit phase. This is where React actually updates the real DOM. Only after this phase is complete does React start running your effects.

Here's the key insight that might surprise you. useEffect runs after the commit phase, not during it. This design choice is brilliant because it ensures that side effects never block the rendering process, keeping your app fast and responsive.

To understand why this matters, imagine you're updating a complex form with hundreds of input fields. If effects ran during the commit phase, every single effect would have to complete before the user could see any visual feedback. By running effects after the commit phase, React ensures that users see updates immediately, while effects run in the background.

This separation also allows React to batch multiple updates together, which is crucial for performance. Instead of running effects one by one, React can collect all the effects that need to run and execute them in an optimized order.

How React Registers Effects: The Internal Hook List

When your component runs, each useEffect call creates an entry in what React calls the "hook list." This is essentially a linked list of all the hooks for that component.

To understand this better, imagine you're managing a restaurant kitchen. Each hook is like a different station. The grill, the salad station, the dessert station. The hook list is like your kitchen's order management system that tracks what each station needs to do and when.

Here's what happens step by step, and why each step matters:

React creates or updates the Effect object - This is like creating a new order ticket for the kitchen station. The effect object contains all the information needed to execute the effect.
It retrieves the previous dependencies - React looks at what the dependencies were during the last render. This is crucial for determining if anything has actually changed.
It compares the new dependencies to the old ones using Object.is() - This is where the magic happens. React doesn't just do a simple equality check. It uses Object.is(), which is more precise than === for certain values like NaN and -0.

If at least one dependency has changed or if there are no dependencies at all, React marks the effect for execution. It's like React is keeping a detailed log of what needs to be done and when.

Here's a simplified version of the logic React uses:

let hasChanged = true;
if (oldDeps) {
  hasChanged = deps.some((dep, i) => !Object.is(dep, oldDeps[i]));
}
if (hasChanged) {
  // Schedule this effect for execution
}

The Object.is() comparison is particularly important because it handles edge cases that regular equality checks miss. For example, Object.is(NaN, NaN) returns true, while NaN === NaN returns false. This precision is crucial for React's dependency tracking to work correctly.

But here's where it gets really interesting. React doesn't just check if dependencies changed. It also considers the effect's "tag" to determine what type of effect it is. Some effects run on every render, some only when dependencies change, and some only on mount and unmount. This tagging system allows React to optimize effect execution based on the effect's intended behavior.

The Commit Phase: When Effects Actually Run

Once React finishes rendering and updating the DOM, it enters what's called the post-commit phase. This is where your effects actually get executed.

Here's the sequence, and why each step is important:

React flushes all pending DOM updates - This ensures that all visual changes are applied to the actual DOM before any effects run. It's like making sure all the paint has dried before you start moving furniture around.
It collects all effects that are marked for execution - React gathers all the effects that need to run from all components in the current render cycle. This batching is crucial for performance.
It runs cleanup functions for any previous effects - Before running new effects, React first cleans up any existing effects. This prevents memory leaks and ensures that old subscriptions don't interfere with new ones.
It executes the new effect callbacks asynchronously - React schedules the effects to run after the current execution stack is complete, often using queueMicrotask() or similar methods.

The background execution part is crucial and deserves a deeper explanation. React often uses queueMicrotask() or similar methods to ensure your effects run after the browser has had a chance to paint the screen. This keeps your UI interactions smooth and responsive.

To understand why this matters, imagine you're watching a movie. If the sound effects played before the visual scenes were ready, the experience would be jarring and confusing. Similarly, if effects ran before the browser could paint the updated UI, users would see inconsistent states or experience stuttering.

The queueMicrotask() mechanism is particularly clever because it ensures effects run in the same event loop turn as the current execution, but after all synchronous code has completed. This means effects run as soon as possible without blocking the main thread, which is essential for maintaining 60fps performance.

The Cleanup Process: React's Memory Management

One of the most beautifully simple parts of useEffect is how it handles cleanup. If your effect returns a function, React automatically stores it as a cleanup function. Then, before running the next effect or when the component unmounts, React calls this cleanup function.

Here's a practical example:

useEffect(() => {
  const id = setInterval(fetchData, 5000);
  return () => clearInterval(id); // This cleanup function runs automatically
}, [data]);

This cleanup mechanism is React's way of preventing memory leaks and stale subscriptions. It's like having an automatic janitor that cleans up after your effects, ensuring nothing gets left behind.

But let's dive deeper into why this cleanup mechanism is so crucial. When you set up a subscription, event listener, or timer, these operations create references that keep objects in memory. Without proper cleanup, these references can build up over time, leading to memory leaks that slow down your application and eventually crash it.

The cleanup function serves as a safety net that ensures these references are properly released. React is particularly smart about when it calls these cleanup functions:

Before the next effect runs - If your dependencies change and the effect needs to run again, React first calls the cleanup function from the previous effect. This prevents overlapping subscriptions or timers.
When the component unmounts - If the component is removed from the tree, React calls all cleanup functions to ensure no lingering references remain.
During error boundaries - If an error occurs during rendering, React still calls cleanup functions to prevent memory leaks even in error scenarios.

The beauty of this system is that you don't have to manually track when to clean up. React handles all the timing for you, ensuring that cleanup happens at exactly the right moment.

Effect Execution Order: The Predictable Sequence

React doesn't run effects in random order. There's a specific, predictable sequence that ensures everything works correctly:

Child effects run before parent effects – This ensures that child updates settle before parent updates
Cleanup functions run before new effects – This prevents overlapping side effects
Unmount cleanups always run – This ensures nothing gets left behind when components are removed

This strict ordering is what makes React's effect system so reliable. You can count on this behavior, which makes debugging much easier.

But why does this ordering matter so much? Let's think about it from a practical perspective. Imagine you have a parent component that manages a list of items, and each item is a child component that subscribes to real-time updates. If the parent's effects ran before the children's effects, the parent might try to process data that the children haven't finished updating yet.

The cleanup-before-new-effects rule is equally important. Consider what would happen if you had a WebSocket connection that needed to be closed before opening a new one. Without this ordering, you might end up with multiple connections running simultaneously, which could lead to duplicate data or race conditions.

React's effect ordering is like having a well-organized assembly line where each step must complete before the next one begins. This predictability is what makes React's effect system so powerful and reliable.

Internal Hook State and the Rules of Hooks

Behind the scenes, React maintains a linked list of hooks attached to each fiber. Every call to useEffect, useState, or useMemo creates an entry in this list.

This is why the Rules of Hooks exist. React relies on the order of hook calls to match hook state correctly between renders. If you change the order of hooks between renders, React loses this mapping and things break. This same principle applies to useState's hook registration system, where the order of hook calls determines how state gets associated with components.

Here's a simplified version of React's internal hook structure:

class Hook {
  constructor() {
    this.state = null; // Stores the effect object
    this.next = null; // Pointer to the next hook
  }
}

To understand why this linked list structure is so important, imagine you're managing a library where each book has a specific shelf position. If you move books around randomly, you'll never be able to find the book you're looking for. Similarly, React uses the order of hook calls to create a "map" of your component's state.

When React renders your component, it walks through this linked list in order, matching each hook call to its corresponding entry. If you change the order of hooks between renders, React's internal "map" gets confused, and it might try to use a useState hook's state for a useEffect hook, leading to unpredictable behavior.

This is why you can't call hooks inside loops, conditions, or nested functions. React needs to be able to predict exactly how many hooks will be called and in what order, so it can maintain this internal mapping correctly.

The linked list structure also allows React to efficiently traverse and update hook state during re-renders. Each hook knows about the next hook in the sequence, making it easy for React to process all hooks in the correct order.

Asynchronous Behavior and Performance Benefits

The asynchronous nature of useEffect provides several performance advantages:

Non-blocking rendering – Effects never delay visual updates
Optimized browser paint – The UI updates before effects run
Batch execution – All effects in a render cycle run together

This is different from useLayoutEffect, which runs synchronously before paint. That's useful for DOM measurements or layout adjustments, but it can potentially block the UI.

Hook Type	Execution Timing	Blocking?
`useEffect`	After paint (asynchronous)	No
`useLayoutEffect`	Before paint (synchronous)	Yes

The asynchronous nature of useEffect is particularly important for maintaining smooth user interactions. When you're scrolling through a long list or typing in a form, you want the UI to respond immediately to your actions. If effects ran synchronously, they could block these interactions, making your app feel sluggish.

The batching of effects is another crucial optimization. Instead of running effects one by one, React collects all effects that need to run and executes them together. This reduces the overhead of switching between different execution contexts and allows React to optimize the overall performance of your application.

Effect Queue and Scheduling Priority

React maintains an effect queue and processes effects in batches after commit. The internal scheduler assigns priorities to ensure the user experience remains smooth even under heavy load.

Priority levels include:

Discrete Event Priority – for direct user interactions (clicks, keypresses)
Continuous Event Priority – for animations and scrolling
Default Event Priority – for normal updates
Idle Priority – for background tasks

This scheduling model allows React to pause or delay effects based on importance, preserving responsiveness even when your app is doing a lot of work.

To understand why this priority system is so important, imagine you're driving a car with multiple systems running simultaneously. You have the engine, the air conditioning, the radio, and the GPS all working at the same time. If the air conditioning suddenly demanded all the car's power, your engine might stall, making the car unusable.

Similarly, React's priority system ensures that critical user interactions (like clicks and keypresses) always get processed first, while less important tasks (like background data fetching) can be delayed or paused if necessary. This prevents your app from becoming unresponsive even when it's doing a lot of work in the background.

The scheduler is particularly clever about how it handles these priorities. It can interrupt lower-priority work to handle higher-priority tasks, then resume the lower-priority work when the higher-priority task is complete. This creates a smooth, responsive experience even under heavy load.

Memory Management and Optimization

React employs several clever optimizations for efficiency:

Effect Reuse – Skips re-running effects when dependencies haven't changed
Automatic Cleanup – Prevents memory leaks by tracking and running cleanup functions
Batching Effects – Runs multiple effects in one cycle to reduce overhead

These design choices let you focus on your logic instead of worrying about lifecycle management.

The effect reuse optimization is particularly complex. React doesn't just check if your dependencies have changed. It also considers the effect's tag to determine what type of optimization to apply. For example, effects with no dependencies are marked to run on every render, while effects with dependencies are marked to run only when those dependencies change.

The automatic cleanup system is equally impressive. React maintains a reference to every cleanup function and ensures they're called at the right time. This prevents memory leaks that could accumulate over time, especially in long-running applications.

These optimizations work together to create a system that's both powerful and efficient. You get the full power of React's effect system without having to worry about the complex lifecycle management that would be required in other frameworks.

Common Misunderstandings About useEffect

Let's clear up some misconceptions that trip up even experienced developers:

"useEffect runs during render." ➜ It doesn't, React only schedules it during render.

"Dependencies cause re-renders." ➜ Dependencies only affect whether the effect re-runs, not whether the component re-renders.

"Cleanup runs instantly." ➜ It runs before the next effect or when the component unmounts.

Understanding these details helps you avoid performance pitfalls and subtle bugs.

The first misconception is particularly common because it's easy to think that effects run immediately when you call them. But React is much more sophisticated than that. During the render phase, React is just collecting information about what effects need to run. The actual execution happens later, after the DOM has been updated.

The second misconception about dependencies is also important to understand. Dependencies don't cause re-renders. They only determine whether an effect should run again. The component might re-render for completely different reasons (like state changes or prop updates), but the effect will only run if its dependencies have actually changed.

The third misconception about cleanup timing is crucial for understanding how React manages effect lifecycles. Cleanup doesn't run immediately when an effect finishes. It runs before the next effect or when the component unmounts. This timing is carefully orchestrated to prevent race conditions and ensure proper cleanup.

Practical Tips for Using useEffect Effectively

Here are some battle-tested tips for getting the most out of useEffect:

Keep dependency arrays accurate and minimal
Separate unrelated effects into different hooks
Use useLayoutEffect only when DOM measurement or synchronous updates are required
Avoid performing state updates or expensive operations directly inside render logic

These tips might seem simple, but they're based on years of experience with React's effect system. Let's dive deeper into why each one matters.

Keeping dependency arrays accurate and minimal is crucial for performance. Every dependency you include will be compared on every render, so including unnecessary dependencies can cause effects to run more often than needed. On the other hand, missing dependencies can lead to stale closures and bugs.

Separating unrelated effects into different hooks makes your code more maintainable and easier to debug. It also allows React to optimize each effect independently, potentially skipping effects that don't need to run.

Using useLayoutEffect carefully is important because it can block the UI. Only use it when you need to measure DOM elements or perform synchronous updates that affect layout. For most use cases, useEffect is the better choice.

Avoiding state updates in render logic prevents infinite loops and performance issues. If you need to update state based on props or other state, use useEffect to handle the side effect properly.

FAQ Section

Why Does My useEffect Run Twice in Development?

This one catches a lot of developers off guard. This is React's Strict Mode in action. In development, React intentionally double-invokes effects to help you catch bugs related to cleanup. Your effect should be written to handle being called multiple times safely.

This behavior is designed to help you write more robust code. In production, effects only run once, but in development, React simulates the component mounting, unmounting, and remounting to ensure your cleanup functions work correctly. It's like React is saying, "Hey, let me make sure your code is bulletproof before you ship it".

Can I Use useEffect for Data Fetching?

Absolutely! useEffect is perfect for data fetching. Just make sure to handle loading states, errors, and cleanup properly. Consider using libraries like React Query for more advanced data fetching needs.

When using useEffect for data fetching, it's important to handle the async nature of the operation. You'll want to use a flag to track whether the component is still mounted, and you'll want to handle errors gracefully. Here's a common pattern that works really well.

useEffect(() => {
  let cancelled = false;

  const fetchData = async () => {
    try {
      const response = await api.getData();
      if (!cancelled) {
        setData(response);
      }
    } catch (error) {
      if (!cancelled) {
        setError(error);
      }
    }
  };

  fetchData();

  return () => {
    cancelled = true;
  };
}, []);

This pattern prevents the classic "Can't perform React state update on unmounted component" warning by using useState's state update mechanisms safely.

What's the Difference Between useEffect and useLayoutEffect?

useEffect runs in the background (asynchronously) after paint, while useLayoutEffect runs immediately (synchronously) before paint. Use useLayoutEffect when you need to measure DOM elements or perform synchronous updates that affect layout.

The key difference is timing. useEffect runs after the browser has painted the screen, which means users see the updated UI immediately. useLayoutEffect runs before the browser paints, which can cause the UI to feel sluggish if the effect takes too long. Think of it as the difference between "do this after the user sees the change" vs "do this before the user sees the change."

Why Do I Need to Include Functions in My Dependency Array?

This is one of the trickiest parts of useEffect. Functions are objects in JavaScript, and they get recreated on every render. If you use a function inside your effect, you need to include it in the dependency array or move it inside the effect to avoid stale closures. This is similar to the stale closure issues with useState, where captured values can become outdated.

This is one of the most common sources of bugs with useEffect. When you use a function from props or state inside your effect, that function might be recreated on every render. If you don't include it in the dependency array, your effect might be using a stale version of the function. It's like trying to use an old phone number that's no longer valid.

Can I Skip the Dependency Array?

You can, but it's usually not what you want. Without a dependency array, your effect runs after every render, which can cause infinite loops or performance issues. Always include the dependencies your effect actually uses.

The dependency array is React's way of knowing when your effect needs to run again. If you omit it, React assumes the effect needs to run after every render, which is rarely what you want. It's like telling React "run this every single time something changes" instead of "run this only when these specific things change."

How Do I Clean Up Subscriptions in useEffect?

Return a cleanup function from your effect. This function will be called before the next effect runs or when the component unmounts. This is perfect for clearing intervals, canceling requests, or removing event listeners.

The cleanup function is crucial for preventing memory leaks. Any subscription, timer, or event listener that you set up in your effect should be cleaned up in the cleanup function. This ensures that your component doesn't leave any lingering references when it unmounts. Think of it as turning off the lights when you leave a room. You don't want to waste energy or leave things running unnecessarily. This cleanup pattern works hand-in-hand with useState's memory management strategies to keep your applications fast.

Conclusion

React's useEffect hook might look simple on the surface, but behind the scenes lies an advanced, fiber-based scheduling system that carefully coordinates when and how your side effects run.

By understanding its internals from dependency comparison to effect cleanup, you gain a mental model that makes your React apps more predictable, efficient, and maintainable. You'll write better effects, debug issues faster, and avoid common pitfalls.

The next time you write a useEffect, you'll know exactly what's happening behind the scenes. And that knowledge? It's going to make you a better React developer. Trust me, once you understand how React manages your effects, you'll never look at useEffect the same way again. Combined with your understanding of useState's internals, you now have a complete picture of how React's hook system works under the hood.