Understand the event propagation mechanism: capture and bubble order analysis

Events are captured first or bubbled first? Cracking the mystery of the event triggering sequence

Event processing is a very important part of web development, and the event triggering sequence is one of the mysteries. In HTML, events are usually propagated by "capturing" or "bubbling". Which should be captured first or bubbled first? This is a very confusing question.

Before answering this question, let’s first understand the “capture” and “bubble” mechanisms of events. Event capturing refers to delivering events layer by layer from the top element to the target node, while event bubbling refers to delivering events layer by layer from the target node to the top element. Both propagation methods play an important role in event processing.

In early browsers, the event propagation sequence was pioneered by Netscape. They believe that the propagation order of events should be from the outermost element to the inner element, and then from the inner element to the outer element. Therefore, the Netscape browser defines the event propagation sequence as event capture and event bubbling.

However, with the popularity of the Internet, Microsoft launched its own IE browser and adopted a different event propagation sequence from Netscape. They believe that the propagation of events should start from the innermost element to the outer element, and then propagate from the outer element to the inner element.

In order to solve this mutual incompatibility problem, W3C has developed a unified standard. According to W3C standards, the order of event propagation should be capturing first and then bubbling. This is the order of propagation currently followed by all modern browsers.

Specifically, when an event occurs on an element, the browser will first perform the event capture phase. In the event capture phase, the browser propagates events from the outermost element to the target element. When the event propagates to the target element, it enters the target phase. In the target phase, the browser executes the event handler bound to the target element. Finally, the event enters the bubbling stage, and the browser will propagate the event from the target element to the outer elements until it reaches the outermost element.

In order to better understand the event propagation sequence, we can demonstrate it through a simple example. Suppose we have an HTML document that contains three nested elements:

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

We bind an event handling function to each element, which is executed in the event capture phase and the bubbling phase respectively. We can achieve this through the following code:

var outer = document.getElementById('outer');
var inner = document.getElementById('inner');
var button = document.getElementById('button');

outer.addEventListener('click', function() {
  console.log('Outer capture');
}, true);

inner.addEventListener('click', function() {
  console.log('Inner capture');
}, true);

button.addEventListener('click', function() {
  console.log('Button capture');
}, true);

outer.addEventListener('click', function() {
  console.log('Outer bubble');
}, false);

inner.addEventListener('click', function() {
  console.log('Inner bubble');
}, false);

button.addEventListener('click', function() {
  console.log('Button bubble');
}, false);

When we click the button, the result of the console output will be:

Outer capture
Inner capture
Button capture
Button bubble
Inner bubble
Outer bubble

The order of event propagation can be clearly seen from the results. First, the browser will execute the event processing functions of the capture phase (Outer capture, Inner capture and Button capture) in order from outside to inside. Next, the browser will execute the event processing functions of the bubbling phase (Button bubble, Inner bubble and Outer bubble) in order from the inside to the outside.

Through this example, we can conclude that in modern browsers, the order of event propagation is to capture first and then bubble. This is mandated by standards set by the W3C.

In the actual development process, we usually use the event bubbling mechanism to handle events. Because the event bubbling mechanism can easily implement event delegation, reduce the number of event processing functions and improve performance. The event capture mechanism is relatively rarely used and is only used in some special circumstances.

In summary, the order of event propagation is to capture first and then bubble. By understanding the propagation mechanism of events, we can better handle events and improve the user experience of web pages.

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