Web Front-endJS TutorialHow to declare JS variable scope? Detailed explanation of scope examples within functionsHow to declare JS variable scope? Detailed explanation of scope examples within functions
The scope of a variable refers to the visibility of the variable, while the life cycle (survival period) examines the variable from another perspective.
The scope of variables in JS is divided into global variables and local variables. Those defined within the function are called local variables, and those defined outside the function are called global variables. ("Things outside a function are called global variables" are relative. The premise discussed here is that variables explicitly declared with var. Variables defined without var within a function are global variables by default. Of course, declaring variables without var is frowned upon. ).
var glob = 4;//函数外声明全局变量
function fun() {
var height = 20; //函数内用var声明的是局部变量
weight = 50; //函数内不用var声明的是全局变量
}
fun();
alert(weight);There is no block-level scope in JS, which is enclosed by curly brackets {}. There is in Java. Write the following code in the main method
public static void main(String... args) {
for(int i=0;i<5;i++) {
}
{
int j=10;
}
int z = 20;
System.out.println(i); // i不可见，语法分析时报错，即编译不通过
System.out.println(j); // j不可见，语法分析时报错，即编译不通过
System.out.println(z); // z可见，输出20
}But if in JS
for(var i=0;i<5;i++) {
}
var obj = {name:"Lily"};
for(var attr in obj) {
}
{
var j=10;
}
alert(i);//输出4，没有块级作用域
alert(attr); //输出name，没有块级作用域
alert(j);//输出10，没有块级作用域This also illustrates a problem. Avoid using a for loop in the global scope and declare variables at the same time, otherwise it will cause global naming range of contamination.
Of course, the let keyword was proposed in JS1.7 to declare variables, which only applies to the scope of the for statement.
for(let i=0;i<5;i++) {
//todo
}
alert(i);//运行时报错，提示i未定义Example of scope of variables within js function:
<html>
<head>
</head>
<body>
<script type="text/javascript">
var a = 5;
var c = 3;
function t(){
var a = 6;
var b = 10;
document.write(a+'-----'+b);
document.write(c);
}
t();
</script>
</body>
</html>Different from PHP, if a variable is to be called in a js function, it will first be searched within the function. For example, in the example, a is equal to 6; but c cannot be found in the function body, so it will look up outside the function body. If it finds it, it will be called. If it cannot find it, it will report an error.
The above is the detailed content of How to declare JS variable scope? Detailed explanation of scope examples within functions. For more information, please follow other related articles on the PHP Chinese website!
Python vs. JavaScript: Which Language Should You Learn?May 03, 2025 am 12:10 AMChoosing Python or JavaScript should be based on career development, learning curve and ecosystem: 1) Career development: Python is suitable for data science and back-end development, while JavaScript is suitable for front-end and full-stack development. 2) Learning curve: Python syntax is concise and suitable for beginners; JavaScript syntax is flexible. 3) Ecosystem: Python has rich scientific computing libraries, and JavaScript has a powerful front-end framework.
JavaScript Frameworks: Powering Modern Web DevelopmentMay 02, 2025 am 12:04 AMThe power of the JavaScript framework lies in simplifying development, improving user experience and application performance. When choosing a framework, consider: 1. Project size and complexity, 2. Team experience, 3. Ecosystem and community support.
The Relationship Between JavaScript, C , and BrowsersMay 01, 2025 am 12:06 AMIntroduction I know you may find it strange, what exactly does JavaScript, C and browser have to do? They seem to be unrelated, but in fact, they play a very important role in modern web development. Today we will discuss the close connection between these three. Through this article, you will learn how JavaScript runs in the browser, the role of C in the browser engine, and how they work together to drive rendering and interaction of web pages. We all know the relationship between JavaScript and browser. JavaScript is the core language of front-end development. It runs directly in the browser, making web pages vivid and interesting. Have you ever wondered why JavaScr
Node.js Streams with TypeScriptApr 30, 2025 am 08:22 AMNode.js excels at efficient I/O, largely thanks to streams. Streams process data incrementally, avoiding memory overload—ideal for large files, network tasks, and real-time applications. Combining streams with TypeScript's type safety creates a powe
Python vs. JavaScript: Performance and Efficiency ConsiderationsApr 30, 2025 am 12:08 AMThe differences in performance and efficiency between Python and JavaScript are mainly reflected in: 1) As an interpreted language, Python runs slowly but has high development efficiency and is suitable for rapid prototype development; 2) JavaScript is limited to single thread in the browser, but multi-threading and asynchronous I/O can be used to improve performance in Node.js, and both have advantages in actual projects.
The Origins of JavaScript: Exploring Its Implementation LanguageApr 29, 2025 am 12:51 AMJavaScript originated in 1995 and was created by Brandon Ike, and realized the language into C. 1.C language provides high performance and system-level programming capabilities for JavaScript. 2. JavaScript's memory management and performance optimization rely on C language. 3. The cross-platform feature of C language helps JavaScript run efficiently on different operating systems.
Behind the Scenes: What Language Powers JavaScript?Apr 28, 2025 am 12:01 AMJavaScript runs in browsers and Node.js environments and relies on the JavaScript engine to parse and execute code. 1) Generate abstract syntax tree (AST) in the parsing stage; 2) convert AST into bytecode or machine code in the compilation stage; 3) execute the compiled code in the execution stage.
The Future of Python and JavaScript: Trends and PredictionsApr 27, 2025 am 12:21 AMThe future trends of Python and JavaScript include: 1. Python will consolidate its position in the fields of scientific computing and AI, 2. JavaScript will promote the development of web technology, 3. Cross-platform development will become a hot topic, and 4. Performance optimization will be the focus. Both will continue to expand application scenarios in their respective fields and make more breakthroughs in performance.
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
MantisBT
Mantis is an easy-to-deploy web-based defect tracking tool designed to aid in product defect tracking. It requires PHP, MySQL and a web server. Check out our demo and hosting services.
VSCode Windows 64-bit Download
A free and powerful IDE editor launched by Microsoft
WebStorm Mac version
Useful JavaScript development tools
mPDF
mPDF is a PHP library that can generate PDF files from UTF-8 encoded HTML. The original author, Ian Back, wrote mPDF to output PDF files "on the fly" from his website and handle different languages. It is slower than original scripts like HTML2FPDF and produces larger files when using Unicode fonts, but supports CSS styles etc. and has a lot of enhancements. Supports almost all languages, including RTL (Arabic and Hebrew) and CJK (Chinese, Japanese and Korean). Supports nested block-level elements (such as P, DIV),
SublimeText3 Linux new version
SublimeText3 Linux latest version
