React Fiber Explained for Frontend Engineers

React Fiber is a reimplementation of the reconciliation algorithm. It allows React to pause, abort, or reuse work, which is why your app stays responsive during big updates.

Why do we need this structure?

Before Fiber, React used a synchronous recursive stack to process the component tree. If you had a massive list of components, the main thread would lock up until the entire tree was finished. That meant janky interactions for the user. Fiber breaks that work into smaller units of work.

But I heard this makes React slower, right?

“Doesn’t breaking things into smaller chunks add overhead compared to doing it all at once?”

You’re right about the overhead. You actually lose raw throughput performance by doing work in smaller slices because the engine has to keep track of state between those slices. But (and this is a big one) throughput isn’t the same as responsiveness. Fiber trades total completion time for a smoother perceived experience.

How does it work under the hood?

Think of the Fiber node as a simple object. It holds the component type, state, and props, plus pointers to its child, sibling, and return nodes. It looks roughly like this:

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function createFiber(tag, pendingProps) {
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  return {
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    tag, // Function component or host component
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    pendingProps,
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    child: null,
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    sibling: null,
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    return: null, // parent
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    stateNode: null
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  };
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}

Each node is a unit of work. React can process these in a loop. When the browser has a free moment, it checks if there is more work to do (thanks to requestIdleCallback or similar scheduling mechanisms). If there’s no time left, it yields back to the browser so animations can run.

Is this really necessary for my app?

“Most of my apps are small and don’t have performance issues, so why care about this?”

You’re probably right that for most CRUD apps, you won’t notice a difference. I’m honestly not sure if every React dev needs to know the internals of how the work-in-progress tree is constructed. However, knowing that React breaks work into chunks helps you avoid “long task” warnings in Lighthouse reports.

If you see performance lag, look at your component hierarchy. If you keep your tree shallow and don’t re-render entire trees on every state change, Fiber will do the heavy lifting for you. It’s a clever way to handle concurrency without needing to be a low-level browser engineer.

What happens when the browser is constantly busy? That’s where things get interesting.