React: The Reconciliation Algorithm
Tags: webdev, javascript, react, frontend
Published: 2023-04-04 07:41:40

In this blog post, we will explore the React reconciliation algorithm, which is the core of how React updates the UI efficiently and predictably. We will see what the algorithm does, how it works, and why it is important for React developers to understand it.

**What is the reconciliation algorithm?**

The reconciliation algorithm is React’s way of efficiently updating the DOM when the application’s state changes. It compares the current component tree (virtual DOM) with the new one, calculates the differences (diff), and then makes the necessary updates to the actual DOM.

The reconciliation algorithm is based on two key assumptions:

- Two elements of different types will produce different trees. For example, if a component renders a <div> element in one state, and a <span> element in another state, React will replace the entire subtree under that component.

- The developer can hint at which child elements may be stable across different renders with a key prop. For example, if a component renders a list of items, each item should have a unique key prop to help React identify which items have changed, moved, or been removed.


**How does the reconciliation algorithm work?**

_The reconciliation algorithm works in three steps:_

1. When a component’s state changes, React triggers a re-render of that component and its children. This creates a new virtual DOM tree.

2. React compares the new virtual DOM tree with the previous one, using a depth-first traversal. It starts from the root node and compares each pair of nodes at each level.

3. If React finds a node with a different type, it replaces the entire subtree with the new one. If React finds a node with the same type, it updates only the changed attributes or styles. If React finds an array of children nodes, it uses the key prop to match each child with its corresponding node in the previous tree, and then applies the same comparison logic.


**Why is the reconciliation algorithm important?**


The reconciliation algorithm is important because it allows React to update the UI in a fast and consistent way. It avoids unnecessary DOM manipulations, which can be costly in terms of performance and user experience. It also ensures that the UI reflects the latest state of the application, without any stale or inconsistent data.

The reconciliation algorithm is not perfect, however. It is based on heuristics that work well in most cases, but not in all. Sometimes, React may update more nodes than necessary, or miss some optimizations that could improve performance. That’s why React provides some tools and techniques to help developers fine-tune the reconciliation process, such as:

- Using functional components instead of class components, which can avoid unnecessary re-renders and improve performance.

- Using hooks such as useMemo and useCallback, which can memoize values and functions and avoid re-computations and re-renders.

- Using custom hooks such as useReducer and useLayoutEffect, which can manage complex state logic and side effects more efficiently.

- Using React.memo and PureComponent, which can prevent re-rendering of components that have not changed their props.

- Using shouldComponentUpdate and React.Fragment, which can control when and how components update and render.

- Using keys properly, which can help React identify which children have changed and avoid unnecessary re-ordering or re-creation of nodes.

**Conclusion**

In this blog post, we have learned about the React reconciliation algorithm, which is the core of how React updates the UI efficiently and predictably. We have seen what the algorithm does, how it works, and why it is important for React developers to understand it. We have also learned about some tools and techniques that can help us fine-tune the reconciliation process and improve performance.

We hope you have enjoyed this blog post and learned something new. If you have any questions or feedback, please feel free to leave a comment below. Thank you for reading!