Why do events bubble up and trigger multiple times?
Why is the event bubbling triggered twice?
Event bubbling is a common phenomenon in web development. It means that when an event on an element is triggered, the event will bubble up from the element, triggering the same event on its parent element in turn. event. However, sometimes we find that an event is triggered twice during the bubbling process. In order to better understand why this happens, we need to start with the principle of event bubbling and analyze it with specific code examples.
The principle of event bubbling is based on the DOM tree structure. In an HTML document, all elements are organized into a tree structure according to their nested relationship. When an event is triggered, the event bubbles up from the element where the event occurred, along its parent element, until it reaches the root element. During the bubbling process, the event will trigger the same event handler on each parent element in turn. The advantage of this design is that event delegation processing can be easily performed and a natural event flow can be achieved.
However, event bubbling will be triggered twice, mainly due to improper binding of the event processing function or imperfect event prevention mechanism. Let's look at a specific code example:
<!DOCTYPE html>
<html>
<head>
<title>事件冒泡示例</title>
</head>
<body>
<div id="outer">
<div id="inner">
<button id="btn">Click me!</button>
</div>
</div>
<script>
var outer = document.getElementById('outer');
var inner = document.getElementById('inner');
var btn = document.getElementById('btn');
outer.addEventListener('click', function() {
console.log('Outer clicked!');
});
inner.addEventListener('click', function() {
console.log('Inner clicked!');
});
btn.addEventListener('click', function() {
console.log('Button clicked!');
event.stopPropagation();
});
</script>
</body>
</html>In this example, we have a div element with two nested levels, and a button nested within the innermost div. We bind a click event handler to each div element and button, and output the corresponding information to the console.
When we click the button, we may expect that "Button clicked!" will be output only once, but in fact we will find that it is output twice. This is because during the event bubbling process, the event will trigger the event handler on each parent element in turn. That is, when the button is clicked, the click event handler on the button will be triggered first, and then the innermost div element will be triggered in turn. And the event handler function on the outermost div element. Since we called the event.stopPropagation() method in the button's event handler, this method will prevent the event from bubbling up. However, calling this method inside the event handler will not prevent subsequent event handlers from executing, so the event handler on the innermost div element will still be triggered once.
To solve this problem, we can use the event.stopImmediatePropagation() method in the event processing function of the button. In addition to preventing the event from bubbling, this method can also prevent subsequent event processing functions from implement. Modify the code as follows:
btn.addEventListener('click', function(event) {
console.log('Button clicked!');
event.stopImmediatePropagation();
});In this way, when we click the button, "Button clicked!" will only be output once.
In summary, event bubbling will be triggered twice, mainly due to improper binding of the event processing function or imperfect event prevention mechanism. We need to correctly use the event.stopPropagation() and event.stopImmediatePropagation() methods to control the bubbling and execution of events. Only by understanding the principles and mechanisms of event bubbling can we better deal with the problems caused by event bubbling and improve the user experience of web applications.
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