What are the knowledge points that explain JS strict mode in detail?
This article has compiled relevant knowledge points and code examples in JS strict mode for everyone. If you are interested, let’s learn together.
The so-called strict mode is actually a string that will not be assigned to any variable "use strict"
If this prompt is given in the global scope, the entire script will use strict mode. You can also turn on strict mode only in the function
1. There are no global variables in strict mode
a="test"
There will be in strict mode Error is reported, normal in non-strict mode
2. Delete variable
var There are three declaration situations
var declared Global variables
var Local variables declared
Global variables declared in eval()
The first and second cases cannot be deleted using delete.
First of all, although the global object declared in the first case is an attribute of windows, this attribute has configurable=false and therefore cannot be deleted.
In the second case, local variables are even worse. No one knows how to delete the attached object
The third case eval() will be discussed later.
Back to the topic. That is to say, in non-strict mode, you can delete variables, but it will fail and return false.
Deleting variables in strict mode will report an error.
3. Object
Under the following circumstances, an error will be reported when operating the object
Assigning a value to a read-only attribute will report an error
Using delete for non-configurable properties will report an error
Adding properties for non-extensible objects will report an error
When using object literals, attribute names must be unique. For example,
var person={
name:"1",
name:"2"
}will default to the second value in non-strict mode, and an error will be reported in strict mode.
4. Function
Strict mode requires that function parameter names must be unique
function（n,n）{
// todo
}In non-strict mode, this function declaration will not report an error. Pass The parameter name can only access the second parameter, and the first parameter must be accessed through arguments.
arguments are also different in the two modes
In non-strict mode, modifying the value of the named parameter will be reflected in the arguments object. In strict mode, the two values are independent.
Eliminated arguments.callee (referencing the function itself) and arguments.caller (referencing the calling function).
In strict mode, function names cannot use js reserved words
**In strict mode, functions can only be declared at the top level of the script and inside the function. Declaring the function in an if statement will cause a syntax error. . **
if(true){
function(){
// 严格模式下报错
}
}5.this
When using the function's apply(), call() in non-strict mode, the null or undefined value passed in will be converted to Global object. In strict mode, the this of a function is always the specified value, no matter what value is specified.
var a="1";
function test(){
console.log(this.a)
}
test.call(null) 在非严格模式下会输出1，严格模式下this就指代的就是null,null没有a属性，就会报错。The above is what I compiled for everyone. I hope it will be helpful to everyone in the future.
Related articles:
How to add mp3 audio files by using vue
How to use FileReader in JS to upload pictures Local preview function
How to set the default selected option based on the select tag?
The above is the detailed content of What are the knowledge points that explain JS strict mode in detail?. For more information, please follow other related articles on the PHP Chinese website!
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
What does WeChat's Do Not Disturb mode do?
Feb 23, 2024 pm 10:48 PM
What does WeChat Do Not Disturb mode mean? Nowadays, with the popularity of smartphones and the rapid development of mobile Internet, social media platforms have become an indispensable part of people's daily lives. WeChat is one of the most popular social media platforms in China, and almost everyone has a WeChat account. We can communicate with friends, family, and colleagues in real time through WeChat, share moments in our lives, and understand each other’s current situation. However, in this era, we are also inevitably faced with the problems of information overload and privacy leakage, especially for those who need to focus or
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
Do Not Disturb Mode Not Working in iPhone: Fix
Apr 24, 2024 pm 04:50 PM
Even answering calls in Do Not Disturb mode can be a very annoying experience. As the name suggests, Do Not Disturb mode turns off all incoming call notifications and alerts from emails, messages, etc. You can follow these solution sets to fix it. Fix 1 – Enable Focus Mode Enable focus mode on your phone. Step 1 – Swipe down from the top to access Control Center. Step 2 – Next, enable “Focus Mode” on your phone. Focus Mode enables Do Not Disturb mode on your phone. It won't cause any incoming call alerts to appear on your phone. Fix 2 – Change Focus Mode Settings If there are some issues in the focus mode settings, you should fix them. Step 1 – Open your iPhone settings window. Step 2 – Next, turn on the Focus mode settings
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
