How to implement event handler in javascript

JavaScript is a scripting language mainly used to achieve dynamic effects and interactivity in Web pages. He can respond to user operations on the page through event handlers, such as clicks, double-clicks, moving the mouse, pressing the keyboard and other events. Implementing event handlers is a very important skill in JavaScript. The following is an introduction to implementing event handlers.

1. Basic knowledge

In JavaScript, event handlers can be implemented through attributes in HTML, properties and methods of DOM objects, and JavaScript functions. The most common of these is to add event handlers directly as attributes of HTML elements.

For example:

<button onclick="myFunction()">点击这里</button>

This HTML code will call the myFunction() function when the user clicks the button. Other common event attributes include onmouseover, onmouseout, onkeydown, onload, etc.

2. Implementation method

In addition to using event handlers as attributes of HTML elements, event handlers can also be implemented through the properties and methods of DOM objects. The following code can implement a button click event similar to the one above:

var btn = document.getElementById("my-btn");
btn.onclick = function(){
    alert("您单击了按钮！");
};

This method adds the event handler to the specified HTML element through JavaScript code, rather than directly using it as an attribute of the HTML element.

Another way is to use the addEventListener() method, which is often considered a best practice. Here is the sample code:

var myButton = document.getElementById("my-button");

// 添加事件监听器
myButton.addEventListener("click", function() {
    alert("您单击了按钮！");
});

This approach is more flexible and allows you to add multiple event listeners to the same element and easily remove them when needed.

3. Event Object

In JavaScript, an event object is an object automatically created when an event occurs, which contains detailed information about the event. The event object is passed as a parameter inside the event handler and can be used to access event properties and methods.

For example, we can use the following code to prevent form submission:

var myForm = document.getElementById("my-form");

myForm.addEventListener("submit", function(event) {
    event.preventDefault(); // 阻止表单提交
});

In this example, the "submit" event is added to the form and prevents the default action of the form (i.e. submitting the form ).

4. Event bubbling

In JavaScript, event bubbling means that when an element triggers an event, its parent element will also receive the same event. Event bubbling can cause problems in a number of situations, such as when both child and parent elements have the same event handler.

To solve this problem, we can use the stopPropagation() method to prevent the bubbling delivery of events. The following is sample code:

var myButton = document.getElementById("my-button");

// 添加事件监听器
myButton.addEventListener("click", function(event) {
    event.stopPropagation(); // 阻止事件冒泡
});

It must be noted that if the same event handler exists on multiple sibling elements, then event delegation will need to be used to solve this problem.

5. Conclusion

Implementing event handlers in JavaScript is a very important skill. Event handlers can be implemented using HTML attributes, properties and methods of DOM objects, JavaScript functions, and the addEventListener() method. The event object is passed as a parameter inside the event handler, allowing access to event information. Event bubbling can be stopped using the stopPropagation() method, and event delegation is needed to solve this problem when the same event handler exists on multiple sibling elements.

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