The Principle of the Virtual DOM Diff Algorithm

The Virtual DOM diff algorithm is a core optimization mechanism in front-end frameworks. When a component's state changes, the framework creates a new Virtual DOM tree, then compares the differences between the new and old trees using the diff algorithm, and finally applies only the changed parts to the real DOM, avoiding a complete re-render.

Core Algorithm Process:

1. Tree Comparison Strategy

   • Hierarchical Comparison: Only nodes at the same level are compared; cross-level comparisons are not performed.
   • Time Complexity Optimization: Traditional tree diff algorithms have O(n³) complexity, which is optimized to O(n) through strategic improvements.
   • Depth-First Traversal: Nodes are compared recursively using depth-first traversal.

2. Node Comparison Rules

   // Three possible outcomes of node comparison
   if (oldNode does not exist) {
     // Case 1: New Node Addition
     Create new node and insert it
   } else if (newNode does not exist) {
     // Case 2: Node Deletion
     Remove old node
   } else if (node types are different) {
     // Case 3: Different Node Types
     Replace old node with new node
   } else {
     // Case 4: Same Node Type, compare attributes and children
     Update node attributes and child nodes
   }
   

3. Key Optimization for List Nodes

   • Problem: When the order of list nodes changes, the absence of keys can lead to a large number of unnecessary re-renders.
   • Solution: Assign unique key attributes to list nodes.
   • Comparison Process:
     • Create a mapping table from old node keys to their positions.
     • Traverse the new nodes, looking for reusable old nodes via their keys.
     • Update the list by moving nodes rather than recreating them.

4. Attribute Update Strategy

   • Compare differences between old and new attributes.
   • Only update the attributes that have changed.
   • Special handling for specific attributes (e.g., className, style, event listeners).

Algorithm Optimization Techniques:

• Component trees of the same type have similar structures, while those of different types are replaced entirely.
• Reduce unnecessary diffs using shouldComponentUpdate or React.memo.
• Use key correctly in list rendering to improve reuse efficiency.

This difference comparison mechanism ensures efficient rendering performance even during large-scale view updates.