A detailed explanation of how to handle event delegates in JavaScript

1. Introduction to the event entrustment

Event delegate refers to binding an event handler to the parent element, rather than directly binding to each child element. Through the event bubble mechanism, the event will eventually trigger the processing function on the parent element, and the parent element can determine the child element that is actually clicked based on the event's target ().

In JavaScript, event delegates are an optimization method that improves performance and simplifies code, especially in dynamically generated elements, avoiding binding event listeners for each element.

2. Why use event delegate

Performance optimization: If there are a large number of similar child elements that require binding events, binding events directly for each child element may cause performance problems. After using event delegates, you only need to bind the event once on the parent element, which can reduce the memory usage.

Dynamic Element Support: If the child elements on the page are dynamically generated (such as those added with JavaScript), traditional event binding methods cannot directly bind events to these new elements, and event delegates can solve this problem.

Concise code: Event delegates can make the code more concise and avoid repeated writing event listeners for each child element.

3. Principle of event delegation

Event delegates rely on JavaScript's event bubble mechanism. Event bubbles are when an event occurs, it starts from the target element, propagates up layer by layer to its parent element, and eventually reaches the document or window.

The key points of event delegation are:

• Bind the event handler on the parent element.
• Gets the child element that actually triggers the event.
• Perform the corresponding operation according to the event target.

For example, when a child element is clicked, the event will bubble to the parent element, and the event handler function on the parent element can get the child element that was actually clicked.

4. Actual application of event delegation

4.1 Example 1: Dynamically generated list item click event

Suppose you have a dynamically generated list item, and when the user clicks on a list item, you need to perform some actions. If each list item is bound to an event handler, performance may be wasted. Here is how to use event delegates to optimize this operation.

HTML Code

<ul >
    <li>Task 1</li>
    <li>Task 2</li>
    <li>Task 3</li>
</ul>

<button >添加Task</button>

JavaScript Code

// Event delegate is bound to parent element <ul>const taskList = ('task-list');

// Listen to click events and use event delegation('click', function(event) {
    // Determine whether the clicked element is a <li>    if (() === 'li') {
        alert('You clicked on the task: ' + );
    }
});

// Dynamically add new tasks('add-task').addEventListener('click', function() {
    const newTask = ('li');
     = 'New Mission';
    (newTask);
});

explain

• The click event handler function is bound on #task-list.
• In the event handling function, determine whether the clicked element is a li element.
• When the li element is clicked, a prompt box will pop up, displaying the content of the task.
• When the "Add Task" button is clicked, a new li element will be generated dynamically, and the event delegation can ensure that the new task item will also respond to the click event.

4.2 Example 2: Form Verification

In a form, there may be multiple input fields, and you need to perform some validation operations when the blur event of each input box occurs. If you bind event handlers directly for each input box, it may cause duplication of code. Using event delegates can effectively simplify the code.

HTML Code

<form >
    <input type="text" name="username" placeholder="Please enter a username" />
    <input type="email" name="email" placeholder="Please enter your email" />
    <button type="submit">submit</button>
</form>

JavaScript Code

const form = ('form');

// Event delegate: bind event to parent element <form>('blur', function(event) {
    // Check whether it is the input box's blur event    if (() === 'input') {
        // Get the name of the input box        const inputName = ;
        const inputValue = ;

        // Simple verification rules: the user name cannot be empty, is the mailbox format correct        if (inputName === 'username' &amp;&amp; !inputValue) {
            alert('Username cannot be empty');
        }
        if (inputName === 'email' &amp;&amp; !/\S+@\S+\.\S+/.test(inputValue)) {
            alert('Please enter a valid email address');
        }
    }
}, true);  // Use the capture stage to monitor

explain

The blur event is bound on the form, and the event delegate is used to handle all input boxes' out-of-focus events.

According to the calculation, which input box triggered the blur event and perform corresponding verification.

This approach avoids binding the blur event listener separately for each input box.

5. Pros and cons of event delegation

advantage

• Performance improvement: Especially in scenarios with dynamic elements or large numbers of elements, the number of event processors is reduced and memory consumption is reduced.
• Reduce redundant code: You can avoid writing duplicate event binding code for each element.
• Support dynamic elements: Newly added to the page can also automatically respond to events.

shortcoming

• Complex event target judgment: Sometimes it is necessary to judge the target element of the event based on , which may make the code slightly complicated, especially when multi-layer nesting needs to be considered during event delivery.
• Performance issues: Event delegates can improve performance, performance may also be affected if the event handler on the parent element is very complex or if there are too many events being listened to.
• Debugging difficulty: Because the event handler function is bound to the parent element, the actual event source may be tracked through the event target during debugging, which may increase the complexity of debugging.

6. FAQs and Optimization

Question 1: If there is () or () in the event handling function, will it affect the delegation?

• () will prevent the event from bubbling, causing the event to fail to reach the event handler of the parent element.
• () blocks the browser's default behavior, but does not prevent events from bubbled. Therefore, the event delegation is still valid.

If stopPropagation() is called in the event processor, the event cannot be captured through the event delegation mechanism.

Question 2: How to avoid the complexity of judging event goals in delegation?

Simplify judgment by adding a specific class name or ID to the target element.

If the event target is complex, consider using the matches() method, which can help determine whether the target element matches a CSS selector.

if (('li')) {
    // Handle events}

Event delegates can make your code more concise and efficient, especially when dealing with a large number of child elements or dynamic elements, which is a very practical way to optimize.

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