事件冒泡是指事件從觸發(fā)元素向上冒泡到文檔根節(jié)點(diǎn)的行為，允許在父元素上處理事件，避免為每個(gè)子元素單獨(dú)綁定監(jiān)聽器。例如點(diǎn)擊嵌套在div中的按鈕時(shí)，事件會(huì)先在按鈕觸發(fā)，再傳播至其父級div和更高層級。多數(shù)事件默認(rèn)冒泡，可通過event.stopPropagation()阻止。事件捕獲則與冒泡相反，從文檔頂層向下傳播，需通過addEventListener的capture參數(shù)啟用。事件委託正是利用冒泡機(jī)制，在父元素上統(tǒng)一處理子元素事件，提升性能並支持動(dòng)態(tài)內(nèi)容管理。

Event delegation, bubbling, and capturing are all part of how events work in JavaScript. Understanding them helps you write more efficient code—especially when dealing with dynamic content or large numbers of elements.

What event bubbling actually means

When an event happens on an element (like a click), it doesn't just stay there. It bubbles up through the DOM tree. That means if you click a button inside a div, the event will first trigger on the button, then on the div, and so on until it reaches the top of the document.

This behavior is useful because it lets you handle events at a higher level instead of attaching listeners to every single child element. For example, imagine a list with dozens of items—you can listen for clicks on the whole list ( <ul></ul> ) and figure out which item was clicked using event.target .

A few things to remember about bubbling:

• Most events bubble by default.
• You can stop this using event.stopPropagation() , but be careful—it can make debugging harder if overused.
• Bubbling happens after the target phase, which is when the event hits the actual element that triggered it.

How event capturing works differently

Capturing is the opposite of bubbling—at least in order. Instead of starting from the target and moving up, capturing starts from the top of the document and moves down toward the target element. This is the phase where parent elements get a chance to respond before the actual target does.

You don't see capturing used as often, but it can be helpful in certain cases—like highlighting a form section when any of its fields are focused.

To use capturing, you have to specify it in the addEventListener call:

 element.addEventListener('click', handler, true);

Or more recently (and clearly):

 element.addEventListener('click', handler, { capture: true });

If you're not sure whether you need capturing, chances are you don't. Most of the time, bubbling handles what you need.

Why event delegation matters

Event delegation is a pattern that takes advantage of bubbling. Instead of setting up individual event listeners on each child element, you attach one listener to a parent and let the event bubble up. Then, you check event.target to see what originally triggered the event.

This has two main benefits:

• Less memory usage, since you're not creating dozens (or hundreds) of listeners.
• Works well with dynamically added or removed elements—you don't have to re-bind events every time the DOM changes.

Let's say you have a table with rows that can be clicked. Here's how you might do it with delegation:

 document.querySelector('table').addEventListener('click', function(e) {
  if (e.target.closest('tr')) {
    console.log('Row clicked:', e.target.closest('tr'));
  }
});

The key here is checking e.target and using something like closest() to find the nearest relevant element. This way, even if your table updates via AJAX or some user action, you don't have to worry about reattaching handlers.

And yes, you could technically use capturing for event delegation too—but again, most people stick with bubbling unless they have a specific reason not to.

So yeah, those three concepts—bubbling, capturing, and delegation—are closely tied together. Bubbling is the default flow, capturing goes the other way, and delegation uses bubbling (usually) to keep your code clean and efficient.

No magic tricks involved. Just understanding how events move through the DOM and how to take advantage of that movement.

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