search
HomeWeb Front-endJS TutorialUnderstanding JavaScript event objects_javascript tips

When an event on the DOM is triggered, an event object event will be generated.

Event object in DOM

DOM-compatible browsers will pass an event object into the event handler. The event object contains properties and methods related to the specific event that created it. The event types of division are different, and the available attribute methods are different. However, all events will have members listed in the table below.

The properties defined by the Level 2 DOM event standard are listed below:

  • bubbles: Returns a Boolean value indicating whether the event is a bubble event type.
  • cancelable: Returns a Boolean value indicating whether the event can have a cancelable default action.
  • currentTarget: Returns the element whose event listener triggered this event.
  • eventPhase: Returns the current phase of event propagation.
  • target: Returns the element that triggered this event (the target node of the event).
  • timeStamp: Returns the date and time the event was generated.
  • type: Returns the name of the event represented by the current Event object.

Listed below are the methods defined by the Level 2 DOM event standard. IE's event model does not support these methods:

  • initEvent(): Initialize the properties of the newly created Event object.
  • preventDefault(): Informs the browser not to perform the default action associated with the event.
  • stopPropagation(): No longer dispatch events.

this, target, currentTarget

Inside the event handler, the object this is always equal to the value of currentTarget, while target only contains the actual target of the event. If the event handler is assigned directly to the target element, this, currentTarget, and target contain the same values. Such as:

var btn = document.querySelector("#btn");
btn.onclick=function () {
  console.log(event.currentTarget === this); //true
  console.log(event.target === this); //true
}

Since the target of the click event is the btn button, these three values ​​​​are equal. If the event handler is in the button's parent node (document.body), then these values ​​are not the same. Such as:

var btn = document.querySelector("#btn");
document.body.onclick=function () {
  console.log(event.currentTarget === document.body); //true
  console.log(this === document.body); //true
  console.log(event.target === btn); //true 因为btn没有注册事件处理程序,所以该click事件就冒泡到了document.body
}

Here, this and currentTarget are both document.body, because the event handler is registered to this element. But the target element is equal to the button element because it is the real target of the click event. Since the button does not have an event handler registered, the click event bubbles up to document.body, where the event can be handled.

1. type

When you need to handle multiple events through a function, you can use the type attribute. Such as:

//获取按钮
var btn = document.querySelector("#btn");
//设置多个事件
var handler = function() {
//检测事件的类型
  switch (event.type) {
    case "click":
      console.log("i click it");
      break;
    case "mouseover":
      console.log("i enter it");
      break;
    case "mouseout":
      console.log("i leave it");
      break;
  }
}
//给响应的事件赋值
btn.onclick = handler;
btn.onmouseover = handler;
btn.onmouseout = handler;

2. preventDefault()

To prevent the default behavior for a specific event, you can use this method. Such as:

var aTags = document.getElementsByTagName("a");
for (var i = 0; i < aTags.length; i++) {
  var currentATag = aTags[i];
  currentATag.onclick = function() {
    event.preventDefault();
  }
};

The above code blocks all a tag hyperlink functions on the web page. It should be noted that only events with the cancelable attribute set to true can use preventDefault() to cancel their default behavior.

3. stopPropagation()

Immediately stop the propagation of events in the DOM hierarchy, that is, cancel further event capturing or bubbling. For example, an event handler added directly to a button can call this method to avoid triggering the event handler registered on document.body. Such as:

var btn = document.getElementById("btn");
btn.onclick = function () {
  console.log("btn clicked");
  // event.stopPropagation();
};
window.onclick = function () {
  console.log("clicked");
};
//单击一下的结果:
//btn clicked
//clicked

Another example:

var btn = document.getElementById("btn");
btn.onclick = function () {
  console.log("btn clicked");
  event.stopPropagation();
};
window.onclick = function () {
  console.log("clicked");
};
//单击一下的结果:
//btn clicked

eventPhase

This attribute is used to determine which stage of the event flow the event is currently in.

  • If it is the capture phase, it is equal to 1;
  • If it is the target object stage, it is equal to 2;
  • If it is the bubbling stage, it is equal to 3;

For example:

var btn = document.getElementById("btn");

document.body.addEventListener("click", function() {
  console.log("bodyListener" + event.eventPhase);
}, true) //捕获阶段

btn.onclick = function() {
  console.log("btn" + event.eventPhase);
} //目标对象阶段,实际上属于冒泡阶段(在btn上)

document.body.onclick = function() {
  console.log("body" + event.eventPhase);
} //冒泡阶段(在body上)

Another example:

var btn = document.getElementById("btn");

document.body.addEventListener("click", function() {
  console.log(event.eventPhase); //1
  console.log(event.currentTarget); //HTMLBodyElement
}, true);

btn.addEventListener("click", function() {
  console.log(event.eventPhase); //2
  console.log(event.currentTarget); //HTMLInputElement
});

document.body.addEventListener("click", function() {
  console.log(event.eventPhase); //3
  console.log(event.currentTarget); //HTMLBodyElement
});

The process is roughly:

document.body capture phase --> btn target object phase --> document.body bubbling phase

The above is about JavaScript event objects, I hope it will be helpful to everyone’s learning.

Statement
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
Is JavaScript Written in C? Examining the EvidenceIs JavaScript Written in C? Examining the EvidenceApr 25, 2025 am 12:15 AM

Yes, the engine core of JavaScript is written in C. 1) The C language provides efficient performance and underlying control, which is suitable for the development of JavaScript engine. 2) Taking the V8 engine as an example, its core is written in C, combining the efficiency and object-oriented characteristics of C. 3) The working principle of the JavaScript engine includes parsing, compiling and execution, and the C language plays a key role in these processes.

JavaScript's Role: Making the Web Interactive and DynamicJavaScript's Role: Making the Web Interactive and DynamicApr 24, 2025 am 12:12 AM

JavaScript is at the heart of modern websites because it enhances the interactivity and dynamicity of web pages. 1) It allows to change content without refreshing the page, 2) manipulate web pages through DOMAPI, 3) support complex interactive effects such as animation and drag-and-drop, 4) optimize performance and best practices to improve user experience.

C   and JavaScript: The Connection ExplainedC and JavaScript: The Connection ExplainedApr 23, 2025 am 12:07 AM

C and JavaScript achieve interoperability through WebAssembly. 1) C code is compiled into WebAssembly module and introduced into JavaScript environment to enhance computing power. 2) In game development, C handles physics engines and graphics rendering, and JavaScript is responsible for game logic and user interface.

From Websites to Apps: The Diverse Applications of JavaScriptFrom Websites to Apps: The Diverse Applications of JavaScriptApr 22, 2025 am 12:02 AM

JavaScript is widely used in websites, mobile applications, desktop applications and server-side programming. 1) In website development, JavaScript operates DOM together with HTML and CSS to achieve dynamic effects and supports frameworks such as jQuery and React. 2) Through ReactNative and Ionic, JavaScript is used to develop cross-platform mobile applications. 3) The Electron framework enables JavaScript to build desktop applications. 4) Node.js allows JavaScript to run on the server side and supports high concurrent requests.

Python vs. JavaScript: Use Cases and Applications ComparedPython vs. JavaScript: Use Cases and Applications ComparedApr 21, 2025 am 12:01 AM

Python is more suitable for data science and automation, while JavaScript is more suitable for front-end and full-stack development. 1. Python performs well in data science and machine learning, using libraries such as NumPy and Pandas for data processing and modeling. 2. Python is concise and efficient in automation and scripting. 3. JavaScript is indispensable in front-end development and is used to build dynamic web pages and single-page applications. 4. JavaScript plays a role in back-end development through Node.js and supports full-stack development.

The Role of C/C   in JavaScript Interpreters and CompilersThe Role of C/C in JavaScript Interpreters and CompilersApr 20, 2025 am 12:01 AM

C and C play a vital role in the JavaScript engine, mainly used to implement interpreters and JIT compilers. 1) C is used to parse JavaScript source code and generate an abstract syntax tree. 2) C is responsible for generating and executing bytecode. 3) C implements the JIT compiler, optimizes and compiles hot-spot code at runtime, and significantly improves the execution efficiency of JavaScript.

JavaScript in Action: Real-World Examples and ProjectsJavaScript in Action: Real-World Examples and ProjectsApr 19, 2025 am 12:13 AM

JavaScript's application in the real world includes front-end and back-end development. 1) Display front-end applications by building a TODO list application, involving DOM operations and event processing. 2) Build RESTfulAPI through Node.js and Express to demonstrate back-end applications.

JavaScript and the Web: Core Functionality and Use CasesJavaScript and the Web: Core Functionality and Use CasesApr 18, 2025 am 12:19 AM

The main uses of JavaScript in web development include client interaction, form verification and asynchronous communication. 1) Dynamic content update and user interaction through DOM operations; 2) Client verification is carried out before the user submits data to improve the user experience; 3) Refreshless communication with the server is achieved through AJAX technology.

See all articles

Hot AI Tools

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress AI Tool

Undress images for free

Clothoff.io

Clothoff.io

AI clothes remover

Video Face Swap

Video Face Swap

Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Tools

SecLists

SecLists

SecLists is the ultimate security tester's companion. It is a collection of various types of lists that are frequently used during security assessments, all in one place. SecLists helps make security testing more efficient and productive by conveniently providing all the lists a security tester might need. List types include usernames, passwords, URLs, fuzzing payloads, sensitive data patterns, web shells, and more. The tester can simply pull this repository onto a new test machine and he will have access to every type of list he needs.

Dreamweaver CS6

Dreamweaver CS6

Visual web development tools

SAP NetWeaver Server Adapter for Eclipse

SAP NetWeaver Server Adapter for Eclipse

Integrate Eclipse with SAP NetWeaver application server.

SublimeText3 Linux new version

SublimeText3 Linux new version

SublimeText3 Linux latest version

SublimeText3 Mac version

SublimeText3 Mac version

God-level code editing software (SublimeText3)