虛擬DOM是一種優(yōu)化真實(shí)DOM更新的編程概念，通過在內(nèi)存中創(chuàng)建與真實(shí)DOM對應(yīng)的樹形結(jié)構(gòu)，避免頻繁直接操作真實(shí)DOM。其核心原理是：1. 數(shù)據(jù)變化時生成新的虛擬DOM；2. 對比新舊虛擬DOM找出最小差異；3. 批量更新真實(shí)DOM以減少重排重繪開銷。此外，使用唯一穩(wěn)定key可提升列表對比效率，而部分現(xiàn)代框架已採用其他技術(shù)替代虛擬DOM。

The Virtual DOM is a programming concept used in JavaScript frameworks like React to optimize how updates are applied to the real DOM. Instead of updating the actual DOM every time there's a change, these libraries create a lightweight copy — the virtual DOM — and use it to calculate the most efficient way to update the real one.

What Exactly Is the Virtual DOM?

The Virtual DOM is a tree-like structure in memory that mirrors the actual DOM. Each element in the real DOM has a corresponding node in the virtual one. When data changes in your app, the framework doesn't immediately touch the real DOM. Instead, it creates a new version of the virtual tree reflecting those changes.

This lets the framework compare the old and new virtual trees efficiently. It figures out the minimal set of changes needed to bring the real DOM up to date, avoiding costly and frequent direct manipulations.

For example:

• You click a button that toggles a list.
• The framework updates the virtual DOM with the new state.
• It compares the updated virtual DOM with the previous version.
• Only the parts that changed (like showing or hiding the list) get updated in the real DOM.

Why Use a Virtual DOM?

Directly manipulating the DOM can be slow because browsers have to recalculate styles, layout, and repaint elements — a process known as reflow and repaint . Even small changes can trigger expensive operations if done repeatedly.

Using the Virtual DOM helps reduce this performance hit by:

• Batching multiple updates into one
• Calculating only what needs to change
• Avoiding unnecessary reflows

Frameworks like React do this automatically. You just update your component's state, and React handles the rest behind the scenes.

It's worth noting that not all frameworks use the Virtual DOM. Vue 3, for example, uses a different technique called "compiler optimization" to achieve similar performance benefits without diffing the entire tree.

How Does Diffing Work?

When the virtual DOM is updated, the framework runs a diffing algorithm to find differences between the previous and new virtual trees. This comparison happens very quickly since it's done in memory, not on the real DOM.

Here's a simplified breakdown:

• Elements are compared node by node.
• If a node changes, the framework marks it for an update.
• If a list changes order, the framework tries to reuse existing nodes instead of creating new ones.
• Once the differences are identified, a patch is applied to the real DOM in one go.

One important detail: using keys (like key={id} in React) helps the algorithm identify which items have changed, been added, or removed. Without keys, the diffing might not work as efficiently, especially in lists.

So when you render a list of items, always try to give each item a stable, unique key based on its identity, not its index.

Not Always Necessary, But Often Helpful

While the Virtual DOM improves performance in many cases, it's not a silver bullet. In apps with minimal DOM updates, the overhead of maintaining a virtual copy may not be worth it. Also, modern browsers and newer frameworks have introduced alternative strategies that can be just as fast or faster.

Still, the Virtual DOM remains a powerful tool for building dynamic UIs without manually optimizing every update. For most developers, especially those using React, it works quietly in the background and delivers solid performance out of the box.

Basically, it's a smart shortcut — not something you need to think about every day, but useful to understand when debugging or optimizing performance.

基本上就這些。

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