What is the concept of reconciliation in React?

React uses a powerful mechanism called reconciliation to efficiently update the DOM (Document Object Model) and ensure optimal performance. Reconciliation is the process by which React determines the most efficient way to update the UI in response to changes in data or component hierarchy.

Reconciliation

When React components are rendered, they generate a tree-like structure called the Virtual DOM (VDOM), which is a lightweight representation of the actual DOM. Whenever there are updates in the component state or props, React performs a process known as reconciliation to compare the previous VDOM with the new VDOM and apply only the necessary changes to the actual DOM.

Reconciliation algorithm

React’s reconciliation algorithm employs a diffing mechanism to identify the differences between the old and new VDOMs efficiently. It traverses the component tree, matching elements and attributes to optimize the rendering process. React reconciles components at different levels, making the necessary updates without re-rendering the entire tree.

Key prop

To enhance the efficiency of reconciliation, React relies on the key prop assigned to each element in an array rendered by a component. The key prop helps React uniquely identify elements, allowing it to determine which elements were added, removed, or moved during the reconciliation process. By providing stable keys, we can ensure that the diffing algorithm works optimally.

Reconciliation process

The reconciliation process follows these main steps:

• Element creation: React creates new elements for any new components or elements encountered during the rendering process.

• Component lifecycle methods: React triggers the appropriate lifecycle methods such as componentWillMount, componentDidMount, componentWillReceiveProps, shouldComponentUpdate, etc., as needed.

• Diffing and reconciliation: React performs a diffing process by comparing the old and new VDOMs, looking for changes, additions, or removals.

In React's reconciliation process, when a state is changed in a component, React will re-render that component and its descendants.

• Updates to the DOM: React applies the necessary updates to the actual DOM, ensuring a minimal number of modifications for optimal performance.

Reconciliation strategies

React offers different reconciliation strategies depending on the type of changes detected during diffing:

• Update strategy: When React detects changes in component props or state, it triggers an update strategy where it re-renders the affected components and their descendants, and updates the corresponding elements in the DOM.

• Reordering strategy: If React finds that elements have been reordered within a list, it performs a reordering strategy, where it reorders the elements in the DOM without re-rendering them.

• Keyed strategy: When React encounters an array of elements with keys, it uses the keyed strategy. It matches the old and new elements by their keys and performs the necessary updates efficiently.

Implementation

To observe the reconciliation process in action, and observe how React updates the DOM accordingly, let’s consider a simple React component that renders a list of items:

In this example, each list item has a unique id assigned as the key prop. React utilizes this key prop during the reconciliation process to efficiently update the list when the items prop changes.

Explanation

• Lines 28–39: We added a componentDidMount lifecycle method to the App component. Inside this method, we simulate an update to the items array after a delay of 3 seconds using setTimeout. The update includes modifying the name of the first item and adding a new item to the list.

Testing

To observe the DOM changes and the reconciliation process in action using the browser's developer tools, follow these steps:

• Run the React application: Run your app by typing npm start in the terminal.

• Open the developer tools: In most browsers, you can access the developer tools by right-clicking anywhere on the page and selecting "Inspect" or "Inspect Element." Alternatively, you can use the keyboard shortcut Ctrl + Shift + I (or Cmd + Option + I on a Mac) to open the developer tools.

• Navigate to the elements/inspector tab: Once the developer tools are open, navigate to the "Elements" or "Inspector" tab. This tab allows you to inspect and analyze the structure of the DOM.

• Select the relevant element: In the Elements/Inspector tab, locate the component or element you want to inspect. In this case, look for the <ul> element generated by the ItemList component. You can select the element by clicking on the icon Select and element at the left top corner of the browser developer tool.

• Trigger the reconciliation: After 3 seconds, the items array will be updated, triggering the reconciliation process. Keep an eye on the developer tools and observe any changes happening to the relevant elements.

  • Modified item: You should see the name of the first <li> element changing from "Item 1" to "Updated Item 1." This change indicates that React has efficiently updated the corresponding DOM node without re-rendering the entire list.

  • New item: Additionally, you should observe a new <li> element being added to the <ul> with the text "New Item." This demonstrates how React reconciles the added element and inserts it into the DOM without affecting the existing elements.

By inspecting the DOM changes during the reconciliation process, you can visualize how React updates the DOM efficiently by only modifying the necessary elements. This helps in optimizing performance and providing a smooth user experience.

Conclusion

Reconciliation is an important mechanism in React that ensures efficient updates to the user interface while minimizing the impact on performance. By using the Virtual DOM and employing diffing algorithms, React intelligently determines the optimal way to update the DOM based on changes in component state or props. Understanding the reconciliation process and utilizing key props appropriately can significantly enhance the rendering efficiency of React applications.