Web Front-end
JS Tutorial
JavaScript Variable Object Part 2: VO in different execution contexts
JavaScript Variable Object Part 2: VO in different execution contexts
Variable objects in different execution contexts
Some operations (such as variable initialization) and behaviors of variable objects are common to all types of execution contexts. From this perspective, it is easier to understand variable objects as abstract basic things. Additional content related to variable objects is also defined in the function context.
抽象变量对象VO (变量初始化过程的一般行为)
║
╠══> 全局上下文变量对象GlobalContextVO
║ (VO === this === global)
║
╚══> 函数上下文变量对象FunctionContextVO
(VO === AO, 并且添加了<arguments>和<formal parameters>)Let’s take a look in detail:
Variable objects in the global context
First, we need to give the global object a clear definition:
The global object (Global object) is created before entering any execution context Object. There is only one copy of this object, and its properties can be accessed anywhere in the program. The life cycle of the global object ends at the moment the program exits.
In the initial creation phase of the global object, Math, String, Date, parseInt are used as its own attributes, and other attributes are initialized. It can also have other additionally created objects as attributes (which can point to the global object itself). For example, in the DOM, the window property of the global object can refer to the global object itself (of course, not all specific implementations are like this):
global = {
Math: <...>,
String: <...>
...
...
window: global //引用自身
};When accessing the properties of the global object, the prefix is usually ignored. This is because Global objects are not directly accessible by name. However, we can still access the global object through this of the global context, and we can also refer to itself recursively. For example, window in DOM. To sum up, the code can be abbreviated as:
String(10); // 就是global.String(10); // 带有前缀 window.a = 10; // === global.window.a = 10 === global.a = 10; this.b = 20; // global.b = 20;
So, back to the variable object in the global context - here, the variable object is the global object itself:
VO(globalContext) === global;
It is very necessary to understand the above conclusion, Based on this principle, only when a correspondence is declared in the global context, we can access it indirectly through the properties of the global object (for example, without knowing the variable name in advance).
var a = new String('test'); alert(a); // 直接访问，在VO(globalContext)里找到："test" alert(window['a']); // 间接通过global访问：global === VO(globalContext): "test" alert(a === this.a); // true var aKey = 'a'; alert(window[aKey]); // 间接通过动态属性名称访问："test"
Variable objects in function context
In function execution context, VO cannot be accessed directly. At this time, the activation object (abbreviated as AO) plays the role of VO.
VO(functionContext) === AO;
The active object is created when entering the function context, and it is initialized through the arguments attribute of the function. The value of the arguments attribute is the Arguments object:
AO = {
arguments: <ArgO>
};The Arguments object is a property of the active object, which includes the following properties:
callee — a reference to the current function
length — the number of parameters actually passed
properties-indexes ( Integer of type string) The value of the attribute is the parameter value of the function (arranged from left to right in the parameter list). The number of elements inside properties-indexes is equal to arguments.length. The value of properties-indexes is shared with the parameters actually passed in.
For example:
function foo(x, y, z) {
// 声明的函数参数数量arguments (x, y, z)
alert(foo.length); // 3
// 真正传进来的参数个数(only x, y)
alert(arguments.length); // 2
// 参数的callee是函数自身
alert(arguments.callee === foo); // true
// 参数共享
alert(x === arguments[0]); // true
alert(x); // 10
arguments[0] = 20;
alert(x); // 20
x = 30;
alert(arguments[0]); // 30
// 不过，没有传进来的参数z，和参数的第3个索引值是不共享的
z = 40;
alert(arguments[2]); // undefined
arguments[2] = 50;
alert(z); // 40
}
foo(10, 20);The code for this example has a bug in the current version of Google Chrome - even if the parameter z is not passed, z and arguments[2] are still shared.
Hot AI Tools
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Undress AI Tool
Undress images for free
Clothoff.io
AI clothes remover
Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!
Hot Article
Hot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
How to implement an online speech recognition system using WebSocket and JavaScript
Dec 17, 2023 pm 02:54 PM
How to use WebSocket and JavaScript to implement an online speech recognition system Introduction: With the continuous development of technology, speech recognition technology has become an important part of the field of artificial intelligence. The online speech recognition system based on WebSocket and JavaScript has the characteristics of low latency, real-time and cross-platform, and has become a widely used solution. This article will introduce how to use WebSocket and JavaScript to implement an online speech recognition system.
WebSocket and JavaScript: key technologies for implementing real-time monitoring systems
Dec 17, 2023 pm 05:30 PM
WebSocket and JavaScript: Key technologies for realizing real-time monitoring systems Introduction: With the rapid development of Internet technology, real-time monitoring systems have been widely used in various fields. One of the key technologies to achieve real-time monitoring is the combination of WebSocket and JavaScript. This article will introduce the application of WebSocket and JavaScript in real-time monitoring systems, give code examples, and explain their implementation principles in detail. 1. WebSocket technology
How to use JavaScript and WebSocket to implement a real-time online ordering system
Dec 17, 2023 pm 12:09 PM
Introduction to how to use JavaScript and WebSocket to implement a real-time online ordering system: With the popularity of the Internet and the advancement of technology, more and more restaurants have begun to provide online ordering services. In order to implement a real-time online ordering system, we can use JavaScript and WebSocket technology. WebSocket is a full-duplex communication protocol based on the TCP protocol, which can realize real-time two-way communication between the client and the server. In the real-time online ordering system, when the user selects dishes and places an order
How to implement an online reservation system using WebSocket and JavaScript
Dec 17, 2023 am 09:39 AM
How to use WebSocket and JavaScript to implement an online reservation system. In today's digital era, more and more businesses and services need to provide online reservation functions. It is crucial to implement an efficient and real-time online reservation system. This article will introduce how to use WebSocket and JavaScript to implement an online reservation system, and provide specific code examples. 1. What is WebSocket? WebSocket is a full-duplex method on a single TCP connection.
JavaScript and WebSocket: Building an efficient real-time weather forecasting system
Dec 17, 2023 pm 05:13 PM
JavaScript and WebSocket: Building an efficient real-time weather forecast system Introduction: Today, the accuracy of weather forecasts is of great significance to daily life and decision-making. As technology develops, we can provide more accurate and reliable weather forecasts by obtaining weather data in real time. In this article, we will learn how to use JavaScript and WebSocket technology to build an efficient real-time weather forecast system. This article will demonstrate the implementation process through specific code examples. We
Simple JavaScript Tutorial: How to Get HTTP Status Code
Jan 05, 2024 pm 06:08 PM
JavaScript tutorial: How to get HTTP status code, specific code examples are required. Preface: In web development, data interaction with the server is often involved. When communicating with the server, we often need to obtain the returned HTTP status code to determine whether the operation is successful, and perform corresponding processing based on different status codes. This article will teach you how to use JavaScript to obtain HTTP status codes and provide some practical code examples. Using XMLHttpRequest
How to use insertBefore in javascript
Nov 24, 2023 am 11:56 AM
Usage: In JavaScript, the insertBefore() method is used to insert a new node in the DOM tree. This method requires two parameters: the new node to be inserted and the reference node (that is, the node where the new node will be inserted).
JavaScript and WebSocket: Building an efficient real-time image processing system
Dec 17, 2023 am 08:41 AM
JavaScript is a programming language widely used in web development, while WebSocket is a network protocol used for real-time communication. Combining the powerful functions of the two, we can create an efficient real-time image processing system. This article will introduce how to implement this system using JavaScript and WebSocket, and provide specific code examples. First, we need to clarify the requirements and goals of the real-time image processing system. Suppose we have a camera device that can collect real-time image data
