What does bubbling event mean?

Bubbling events mean that in the browser, when an event occurs on an element, the event will be passed to the parent element in order from the inside to the outside until it is passed to the document root element. This propagation process is similar to the process of bubbles rising in water, so it is called event bubbling. The bubbling event mechanism provides a convenient and flexible way to handle events, reducing the duplication of code, while also increasing the readability and maintainability of the code. You need to pay attention to terminating the propagation of events at the appropriate time to ensure Prevent unexpected results.

The operating system for this tutorial: Windows 10 system, DELL G3 computer.

Bubbling events mean that in the browser, when an event occurs on an element, the event will be passed to the parent element in order from the inside to the outside until it is passed to the document root element ( window object). This propagation process is similar to the process of bubbles rising in water, so it is called event bubbling.

In the browser, each HTML element forms a nested hierarchy, and each element can receive and handle events. When an event is triggered on an element, such as a click event or a mouse movement event, the browser will first trigger the corresponding event callback function on the element, and then propagate it to the parent element in turn until it is passed to the root element of the document.

In order to better understand the process of bubbling events, we can give an example.

Consider the following HTML code snippet:

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

We added a click event listener to the `inner` element:

const innerElement = document.getElementById("inner");
innerElement.addEventListener("click", function(event) {
  console.log("Inner element clicked!");
});

When we click on the "Click me" text , the event is first triggered on the `inner` element, and then starts to propagate upward. In this example, the event is passed to the `middle` element, then to the "outer" element, and finally to the root element of the document (the `window` object). On each element, there can be a listener for the corresponding event, and we can add our own event handling functions on these elements.

So, if we also add a click event listener to the `middle` element:

const middleElement = document.getElementById("middle");
middleElement.addEventListener("click", function(event) {
  console.log("Middle element clicked!");
});

When we click on the "Click me" text, the event is first on the `inner` element Trigger and execute the corresponding callback function. The event will then continue to propagate to the `middle` element and execute its callback function. The final event is propagated to the `outer` element and the `window` object, but since there are no corresponding event listeners on these two elements, the event propagation ends here.

The existence of the bubbling event mechanism has many benefits.

First of all, it provides a convenient way to handle similar events. For example, instead of adding event listeners repeatedly on each child element, we can add the same click event listener to the parent element. This way, when we click on any child element, the event will bubble up to the parent element, thereby executing the callback function on the parent element. Doing so can reduce the amount of code and improve the readability and maintainability of the code.

Secondly, the bubbling event mechanism also makes event processing more flexible. We can choose to intercept the event during the bubbling process and cancel the propagation as needed. Event propagation can be stopped by calling the `stopPropagation()` method of the event object. This can be useful in certain situations to avoid unexpected behavior when handling multiple elements with the same event.

In addition, the bubbling event mechanism can better support event delegation. Event delegation is a common technique for managing event handling on child elements by adding event listeners to parent elements. When elements are added or removed, event listeners do not need to change accordingly. This is useful in scenes with dynamically generated elements.

Of course, the bubbling event mechanism may also cause some problems. For example, when both the child element and the parent element have the same event listener, the event triggers both callback functions at the same time. It may also produce unexpected results because event bubbling overrides the element's default behavior. Therefore, in actual development, we need to judge whether to use the event bubbling mechanism according to the specific situation, or to terminate the propagation of events at the appropriate time.

In summary, bubbling events refer to the process by which events are passed along the element hierarchy from the inside out in the browser. It provides a convenient and flexible way to handle events, reducing code duplication while also increasing code readability and maintainability. However, we need to take care to terminate the propagation of events at the appropriate time to prevent unexpected results.

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