


A brief introduction to JavaScript variables and data types_Basic knowledge
JavaScript data type:
One of the most fundamental features of a programming language is the set of data types it supports. These are the types of programming language values that can be represented and manipulated.
JavaScript allows three basic data types:
- Numbers such as 123, 120.50, etc.
- Strings such as "This text string" etc.
- Boolean type, such as true or false.
JavaScript also defines two data types: null and undefined, each of which only limits a single value.
In addition to these basic data types, JavaScript supports composite data types called objects. We'll see object details covered in a separate chapter.
Note: Java does not make a distinction between integer and floating point values. All numbers in JavaScript are represented as floating point values. JavaScript represents numbers using the 64-bit floating point format defined by the IEEE 754 standard.
JavaScript variables:
Like many other programming languages, JavaScript has variables. Variables can be thought of as named containers. You can put data into these containers and then simply name the containers with reference to that data.
To use a variable in a JavaScript program, it must be declared. Variables are declared using the var keyword as follows:
<script type="text/javascript"> <!-- var money; var name; //--> </script>
You can also declare multiple variables with the same var keyword as follows:
<script type="text/javascript"> <!-- var money, name; //--> </script>
The value stored in a variable is called the initialization of the variable. It can be initialized when a variable is created or updated. It requires a variable, as shown below:
For example, you can create a variable named money and a value of 2000.50, and then assign it to it. For another variable, you can assign a value when initializing as follows:
<script type="text/javascript"> <!-- var name = "Ali"; var money; money = 2000.50; //--> </script>
Note: Use the var keyword only for declaration or initialization. Once a variable name is declared it lives throughout the document. No need to redeclare the same variable twice.
JavaScript is a typed language. This means that JavaScript variables can hold values of any data type. Unlike many other languages, you don't have to tell JavaScript in a variable declaration what type of value the variable will hold. The value type of a variable can be changed during the execution of the program and JavaScript will do so automatically.
JavaScript variable scope:
The scope of a variable is the area in the program where it is defined. JavaScript variables will only have two categories.
- Global variables: Global variables have global scope, which means that they are defined everywhere in JavaScript code.
- Local variables: A local variable will only be visible in the function where it is defined. Function parameters are local functions.
In the body of a function, local variables take precedence over global variables with the same name. If you declare a global variable with the same name as a local variable or a function parameter, you can effectively hide the global variable. Let’s give an example below:
<script type="text/javascript"> <!-- var myVar = "global"; // Declare a global variable function checkscope( ) { var myVar = "local"; // Declare a local variable document.write(myVar); } //--> </script>
This will produce the following results:
local
JavaScript variable name:
Although variable naming in JavaScript maintains the following rules.
- You should not use any reserved JavaScript keywords as variable names. These keywords are mentioned in the next section. For example, break or boolean variable names are not valid.
- JavaScript variable names should not start with a number (0-9). They must be lettered or underlined. For example, 123test is an invalid variable name, but _123test is a valid one.
- JavaScript variable names are case-sensitive. For example, Name and name are two different variables.
JavaScript reserved words:
The following are reserved words in JavaScript. They cannot be used with JavaScript variables, functions, methods, loop tags, or any object names.

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.

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 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.

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'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.

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.

Understanding how JavaScript engine works internally is important to developers because it helps write more efficient code and understand performance bottlenecks and optimization strategies. 1) The engine's workflow includes three stages: parsing, compiling and execution; 2) During the execution process, the engine will perform dynamic optimization, such as inline cache and hidden classes; 3) Best practices include avoiding global variables, optimizing loops, using const and lets, and avoiding excessive use of closures.

Python is more suitable for beginners, with a smooth learning curve and concise syntax; JavaScript is suitable for front-end development, with a steep learning curve and flexible syntax. 1. Python syntax is intuitive and suitable for data science and back-end development. 2. JavaScript is flexible and widely used in front-end and server-side programming.


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

SAP NetWeaver Server Adapter for Eclipse
Integrate Eclipse with SAP NetWeaver application server.

Dreamweaver Mac version
Visual web development tools

SublimeText3 English version
Recommended: Win version, supports code prompts!

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.

WebStorm Mac version
Useful JavaScript development tools