search
HomeWeb Front-endFront-end Q&AWhat are constants in javascript

What are constants in javascript

Nov 24, 2021 pm 04:48 PM
javascriptconstant

In JavaScript, variables that cannot be modified once defined are called constants. Constants represent some fixed data and are values ​​that cannot be modified. The keyword for defining a constant is const. It must be initialized when defining, and the value cannot be modified after initialization. The syntax is "const variable name = value;".

What are constants in javascript

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

In JavaScript, variables that cannot be modified once defined are called constants.

Constants represent some fixed data and cannot be modified; constants can only be read and cannot be modified.

The keyword of constant is const:

const can define one or more constants, which must be initialized when defining, and the value cannot be modified after initialization.

Syntax:

const 变量名=值;
const 变量名1=值1,变量名2=值3,...,变量名n=值n;

Note: Constants and variables are containers used to store data, but the value of the constant cannot be changed during the running of the program, otherwise an error will be reported at runtime. .

const NUM = 666;
// NUM = 888; //尝试修改NUM这个常量的取值,会发现报错了,常量一旦定义就不能再去改变值了
console.log(NUM);

Extended knowledge: Classification of constants in JavaScript

Integer constants

Integer constants are actually positive numbers. Any integer written in JavaScript is an integer constant.

1 / 666 / 99

Real constants

Real constants are actually decimals. Any decimal written in JavaScript is a real constant

3.14 / 6.66

String constant

  • String constant is actually using single quotes or double quotes The content enclosed in quotation marks is called a string constant

  • 'a'

  • 'abc'

  • “1”

  • “BNTang”

  • Note: No matter how many are enclosed in single quotes or double quotes characters, in JavaScript they are all string constants

##Boolean constants

  • Boolean constants are actually true or false , expressed through true and false in JavaScript

  • In JavaScript, Boolean constants have only two values, true (true) or false (false)

Custom constants

Newly added in ES6

const 常量名称 = 常量取值;

[Related recommendations:

javascript learning tutorial]

The above is the detailed content of What are constants in javascript. For more information, please follow other related articles on the PHP Chinese website!

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
Optimizing Performance with useState() in React ApplicationsOptimizing Performance with useState() in React ApplicationsApr 27, 2025 am 12:22 AM

useState()iscrucialforoptimizingReactappperformanceduetoitsimpactonre-rendersandupdates.Tooptimize:1)UseuseCallbacktomemoizefunctionsandpreventunnecessaryre-renders.2)EmployuseMemoforcachingexpensivecomputations.3)Breakstateintosmallervariablesformor

Sharing State Between Components Using Context and useState()Sharing State Between Components Using Context and useState()Apr 27, 2025 am 12:19 AM

Use Context and useState to share states because they simplify state management in large React applications. 1) Reduce propdrilling, 2) The code is clearer, 3) It is easier to manage global state. However, pay attention to performance overhead and debugging complexity. The rational use of Context and optimization technology can improve the efficiency and maintainability of the application.

The Impact of Incorrect Keys on React's Virtual DOM UpdatesThe Impact of Incorrect Keys on React's Virtual DOM UpdatesApr 27, 2025 am 12:19 AM

Using incorrect keys can cause performance issues and unexpected behavior in React applications. 1) The key is a unique identifier of the list item, helping React update the virtual DOM efficiently. 2) Using the same or non-unique key will cause list items to be reordered and component states to be lost. 3) Using stable and unique identifiers as keys can optimize performance and avoid full re-rendering. 4) Use tools such as ESLint to verify the correctness of the key. Proper use of keys ensures efficient and reliable React applications.

Understanding Keys in React: Optimizing List RenderingUnderstanding Keys in React: Optimizing List RenderingApr 27, 2025 am 12:13 AM

InReact,keysareessentialforoptimizinglistrenderingperformancebyhelpingReacttrackchangesinlistitems.1)KeysenableefficientDOMupdatesbyidentifyingadded,changed,orremoveditems.2)UsinguniqueidentifierslikedatabaseIDsaskeys,ratherthanindices,preventsissues

Common Mistakes to Avoid When Working with useState() in ReactCommon Mistakes to Avoid When Working with useState() in ReactApr 27, 2025 am 12:08 AM

useState is often misused in React. 1. Misunderstand the working mechanism of useState: the status will not be updated immediately after setState. 2. Error update status: SetState in function form should be used. 3. Overuse useState: Use props if necessary. 4. Ignore the dependency array of useEffect: the dependency array needs to be updated when the state changes. 5. Performance considerations: Batch updates to states and simplified state structures can improve performance. Correct understanding and use of useState can improve code efficiency and maintainability.

React's SEO-Friendly Nature: Improving Search Engine VisibilityReact's SEO-Friendly Nature: Improving Search Engine VisibilityApr 26, 2025 am 12:27 AM

Yes,ReactapplicationscanbeSEO-friendlywithproperstrategies.1)Useserver-siderendering(SSR)withtoolslikeNext.jstogeneratefullHTMLforindexing.2)Implementstaticsitegeneration(SSG)forcontent-heavysitestopre-renderpagesatbuildtime.3)Ensureuniquetitlesandme

React's Performance Bottlenecks: Identifying and Optimizing Slow ComponentsReact's Performance Bottlenecks: Identifying and Optimizing Slow ComponentsApr 26, 2025 am 12:25 AM

React performance bottlenecks are mainly caused by inefficient rendering, unnecessary re-rendering and calculation of component internal heavy weight. 1) Use ReactDevTools to locate slow components and apply React.memo optimization. 2) Optimize useEffect to ensure that it only runs when necessary. 3) Use useMemo and useCallback for memory processing. 4) Split the large component into small components. 5) For big data lists, use virtual scrolling technology to optimize rendering. Through these methods, the performance of React applications can be significantly improved.

Alternatives to React: Exploring Other JavaScript UI Libraries and FrameworksAlternatives to React: Exploring Other JavaScript UI Libraries and FrameworksApr 26, 2025 am 12:24 AM

Someone might look for alternatives to React because of performance issues, learning curves, or exploring different UI development methods. 1) Vue.js is praised for its ease of integration and mild learning curve, suitable for small and large applications. 2) Angular is developed by Google and is suitable for large applications, with a powerful type system and dependency injection. 3) Svelte provides excellent performance and simplicity by compiling it into efficient JavaScript at build time, but its ecosystem is still growing. When choosing alternatives, they should be determined based on project needs, team experience and project size.

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

mPDF

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),

ZendStudio 13.5.1 Mac

ZendStudio 13.5.1 Mac

Powerful PHP integrated development environment

Dreamweaver CS6

Dreamweaver CS6

Visual web development tools

MantisBT

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.

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.