DOMでのイベントの泡立ちとキャプチャとは何ですか？

事件捕获和冒泡是DOM中事件传播的两个阶段，捕获是从顶层向下到目标元素，冒泡是从目标元素向上传播到顶层。1. 事件捕获通过addEventListener的useCapture参数设为true实现；2. 事件冒泡是默认行为，useCapture设为false或省略；3. 可使用event.stopPropagation()阻止事件传播；4. 冒泡支持事件委托，提高动态内容处理效率；5. 捕获可用于提前拦截事件，如日志记录或错误处理。了解这两个阶段有助于精确控制JavaScript响应用户操作的时机和方式。

So you've heard about event bubbling and capturing in the DOM, but what do they really mean? In short, they're two phases of how events travel through the DOM tree when something happens — like a click or keypress. Understanding them helps you control exactly when and where your JavaScript code responds to user actions.

What’s the difference between bubbling and capturing?

In simple terms:

• Event capturing is the phase where the event goes from the top of the DOM (like window or document) down to the target element.
• Event bubbling is the opposite: the event starts at the target element and then "bubbles up" through its parent elements to the top of the document.

Most people are familiar with bubbling because that's usually the default behavior. For example, if you have a <button> inside a <div>, and both have click handlers, clicking the button will first trigger its handler, then the div's, and so on.

How do I use capturing or bubbling in practice?

When you add an event listener using addEventListener, you can specify whether it should run during the capturing phase or the bubbling phase.

Here's the syntax:

element.addEventListener('click', function, useCapture);

• If useCapture is true, the handler runs during the capturing phase.
• If false (or omitted), it runs during bubbling.

Let’s say you have this structure:

<div id="outer">
  <button id="inner">Click me</button>
</div>

And this JS:

document.getElementById('outer').addEventListener('click', () => {
  console.log('Outer clicked - bubbling');
}, false);

document.getElementById('outer').addEventListener('click', () => {
  console.log('Outer clicked - capturing');
}, true);

When you click the button, the output would be:

1. “Outer clicked - capturing” (because capturing happens before bubbling)
2. Then whatever handler is on the button itself
3. Then “Outer clicked - bubbling”

This gives you fine-grained control over the order of execution.

When would I actually need to care about this?

You might not always need to think about capturing or bubbling, but here are a few common situations where it matters:

• Stopping event propagation: Sometimes you don’t want an event to bubble up. For example, if you have a dropdown menu and clicking inside it shouldn't close the menu. Use event.stopPropagation() to stop it from going further.

• Delegating events: Event bubbling is the reason event delegation works. Instead of attaching a handler to every list item, you can attach one to their container and let events bubble up. This is efficient for dynamic content.

• Capturing for early interception: Rarely used, but useful in some edge cases. For instance, logging all clicks before they reach the target, or handling errors very early.

A few things to watch out for

• stopPropagation() affects both phases unless called during capturing.
• Some events don’t bubble (like focus), so check documentation.
• Bubbling doesn’t go beyond window; only certain events make it all the way up.
• The third parameter (useCapture) in addEventListener is easy to forget — but powerful when needed.

基本上就这些.

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