In some programming languages like C, each piece of code within curly braces has its own scope, and variables are not visible outside the code segment where they are declared. We call this block-level scope ( block scope), and there is no block-level scope in JavaScript. Instead, JavaScript uses function scope: a variable is defined within the body of the function in which it is declared, as well as within the body of any functions within which it is nested. In the following code, i, j and k defined in different positions are all defined in the same scope
function text(o)
{
var i=0;
alert(typeof o);
If(typeof o == "string")
{
var j=0; for(var k=0;k alert(k);//Output 0-9
alert(k);//output 10
}
alert(j);//output 0
}
The function scope of JavaScript means that all variables declared inside the function are always visible within the function body. Interestingly, this means that the variable is already available before it is even declared. This feature of JavaScript is informally called hoisting, that is, all variables declared in the JavaScript function body (not involving assignment) are "advanced" to the top of the function body. Look at the following code
alert(global);//undefined
var global="hello QDao";
alert(global);//hello QDao
}
Due to the characteristics of function scope, local variables are always defined in the entire function body, which means that variables within the function body cover global variables with the same name. However, local variables are not actually assigned until the program executes the var statement. Therefore, the above process is equivalent to: "advancing" the variable declaration within the function to the top of the function body, while leaving the variable initialization at the original position. :
var global;
alert(global);//undefined
global="hello QDao";
alert(global);//hello QDao
}

The shift from C/C to JavaScript requires adapting to dynamic typing, garbage collection and asynchronous programming. 1) C/C is a statically typed language that requires manual memory management, while JavaScript is dynamically typed and garbage collection is automatically processed. 2) C/C needs to be compiled into machine code, while JavaScript is an interpreted language. 3) JavaScript introduces concepts such as closures, prototype chains and Promise, which enhances flexibility and asynchronous programming capabilities.

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JavaScript's applications in the real world include server-side programming, mobile application development and Internet of Things control: 1. Server-side programming is realized through Node.js, suitable for high concurrent request processing. 2. Mobile application development is carried out through ReactNative and supports cross-platform deployment. 3. Used for IoT device control through Johnny-Five library, suitable for hardware interaction.

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The latest trends in JavaScript include the rise of TypeScript, the popularity of modern frameworks and libraries, and the application of WebAssembly. Future prospects cover more powerful type systems, the development of server-side JavaScript, the expansion of artificial intelligence and machine learning, and the potential of IoT and edge computing.

JavaScript is the cornerstone of modern web development, and its main functions include event-driven programming, dynamic content generation and asynchronous programming. 1) Event-driven programming allows web pages to change dynamically according to user operations. 2) Dynamic content generation allows page content to be adjusted according to conditions. 3) Asynchronous programming ensures that the user interface is not blocked. JavaScript is widely used in web interaction, single-page application and server-side development, greatly improving the flexibility of user experience and cross-platform development.


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