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The difference and application of event bubbling and event capturing

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Jan 13, 2024 am 08:07 AM

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The difference and application of event bubbling and event capturing

The difference and application between event bubbling and event capturing

Event bubbling and event capturing are two mechanisms for event delivery in HTML DOM. When developing web applications, understanding their differences and applications can help us better understand the behavior of events and choose the appropriate delivery mechanism based on actual needs.

Event Bubbling
Event bubbling means that when an event on an element is triggered, it will propagate to its parent element layer by layer until it propagates to The root node of the DOM tree. In other words, the event will bubble up from the triggered element to the parent element.

For example, when the button in the following HTML code is clicked, the click event will bubble up to its parent elements div and body in turn:

<body>
    <div>
        <button>Click Me</button>
    </div>
</body>

In JavaScript, use addEventListener Method to register event listeners and capture events. Use the third parameter to specify whether to use event bubbling or event capture delivery mechanism. If the third parameter is not specified or set to false, the event bubbling delivery mechanism will be used.

The following is a code example using the event bubbling delivery mechanism:

document.querySelector('button').addEventListener('click', function() {
    console.log('Button clicked');
});

document.querySelector('div').addEventListener('click', function() {
    console.log('Div clicked');
});

document.querySelector('body').addEventListener('click', function() {
    console.log('Body clicked');
});

When we click the button, 'Button clicked', 'Div clicked' and 'Body clicked' will be printed out in sequence. .

Event Capturing
Event capturing means that when an event on an element is triggered, it will propagate downward from the root node of the DOM tree until it is triggered. Event elements. In other words, the event will be captured starting from the root node of the DOM tree until the element that triggered the event.

If you want to use the event capture delivery mechanism, you can set the third parameter of the addEventListener method to true. For example:

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

document.querySelector('div').addEventListener('click', function() {
    console.log('Div clicked');
}, true);

document.querySelector('body').addEventListener('click', function() {
    console.log('Body clicked');
}, true);

When we click the button, 'Body clicked', 'Div clicked' and 'Button clicked' will be printed out in sequence. As you can see, the event is captured starting from the root node of the DOM tree, and then propagated to the elements where the event is triggered in turn.

3. Practical application

Understanding the difference between event bubbling and event capturing can help us decide how to deal with event delivery issues in actual development, thereby achieving more flexible interactive functions.

For example, when we have multiple nested elements in a complex page and want to click on one of the elements, only the click event of that element will be triggered. You can choose to use event capture to handle it.

On the other hand, if we want to click on an element and also trigger the click event of its parent element, we can choose to use the event bubbling delivery mechanism.

At the same time, we can also use the stopPropagation() method of the event object to stop further delivery of the event. For example, when we call the stopPropagation() method when a button is clicked, we can prevent the event from continuing to pass up or down.

Summary:
Event bubbling and event capturing are two mechanisms for event delivery in HTML DOM. Understanding their differences and applications can help us better handle event delivery issues and achieve more flexible interactive functions. Choose the appropriate delivery mechanism according to actual needs, and combine the methods of event objects to control the delivery of events.

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