How Do React Hooks Work Under the Hood? A Deep Dive into the Linked List Mechanism

🔍 How Do React Hooks Work Under the Hood?

A Deep Dive into the Singly Linked List Mechanism

React Hooks aren’t magic—they’re built upon a well-defined data structure and a deterministic algorithm. If you’ve ever wondered why you can’t call Hooks conditionally and how React “knows” which useState corresponds to which state, this article is for you.

🧠 Hooks as a Singly Linked List

Internally, React represents each functional component with a Fiber node. This node contains a memoizedState property, which holds a singly linked list of Hooks. Each node in this list represents a single Hook, containing properties like the current state (memoizedState) and a reference to the next Hook (next).

During rendering, React traverses this list in the exact order that Hooks are called within the component. Therefore, the order of Hook calls must remain consistent across renders. Calling Hooks conditionally (e.g., inside if statements) can disrupt this order, leading to errors and unpredictable behavior.

⚙️ How React Manages Hooks

On the initial render (mounting), React constructs a new linked list of Hooks, appending each to the memoizedState of the Fiber node. For subsequent renders (updates), React traverses the existing list, updating the state and effects as needed.

Each Hook is an object structured as follows:

{
  memoizedState: ..., // current state value
  baseState: ...,     // initial state value
  queue: ...,         // update queue
  next: ...,          // reference to the next Hook
}

This structure allows React to efficiently manage state and side effects in functional components.

🚫 Why Can’t Hooks Be Called Conditionally?

React relies on the order of Hook calls to associate state and effects correctly. If a Hook is called conditionally, the order can change between renders, causing React to misalign state assignments. This leads to bugs and inconsistent component behavior.

Example

function MyComponent({ condition }) {
  const [state1, setState1] = useState(0);
  if (condition) {
    const [state2, setState2] = useState(0); // ❌ This is problematic
  }
  // ...
}

If condition changes between renders, the number and order of Hook calls change, leading to errors.

🧩 Why Use a Linked List Instead of an Array?

• Efficiency: Linked lists allow for efficient insertion and deletion of elements without the need to reindex, which is beneficial for dynamic Hook management.
• Predictable Traversal: The sequential nature of linked lists ensures that Hooks are processed in the exact order they are defined, which is crucial for maintaining consistent state associations.
• Flexibility: Linked lists facilitate the addition of new types of Hooks and make it easier to manage complex Hook interactions.

📝 Summary

• React stores Hooks as a singly linked list in the memoizedState property of the Fiber node.
• The order of Hook calls is critical and must remain consistent across renders.
• Avoid calling Hooks conditionally to prevent errors and ensure predictable behavior.
• The linked list structure provides an efficient and flexible way to manage state and side effects in functional components.

Understanding this underlying mechanism can help you write more predictable and efficient React components.