The React Reconciliation Algorithm
19 January, 2023
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What is reconciliation in React?
In React, reconciliation is the process by which the virtual DOM (a lightweight in-memory representation of the actual DOM) compares the app's current state to the desired state and then updates the actual DOM to match. This process allows React to efficiently update the user interface by only making changes to the parts of the DOM that need to be updated rather than re-rendering the entire app. The algorithm that React uses to perform reconciliation is called the diff algorithm.
The diffing algorithm
Sure! The basic idea behind the diff algorithm is to compare the virtual DOM trees of the current and desired states and then make the minimum number of changes necessary to the actual DOM to align with the desired state.
The algorithm starts by comparing the root elements of the two trees. If they are different, it replaces the entire subtree rooted at the current root element with a new subtree corresponding to the desired root element.
If the root elements are the same, the algorithm compares the children of the current and desired root elements. It does this by matching each child in the current tree with a child in the desired tree with the same key (if the child has a key prop) or by matching them by position (if the child does not have a key prop).
For each child with the same key or position in both trees, the algorithm recursively compares their subtrees in the same way.
Suppose the algorithm finds that the current and desired trees have different children at a particular position. In that case, it replaces the entire subtree rooted at that child in the current tree with the corresponding subtree from the desired tree.
In this way, the algorithm can efficiently update the actual DOM to match the desired state while minimizing the number of changes made to the DOM, which helps to improve performance.
It's worth noting that this is a high-level explanation, the actual implementation is slightly more complex and nuanced, but this gives a general idea.
