[JavaScript Tutorial] JavaScript Scope

JavaScript Scope

A collection of scope accessible variables.

JavaScript scope

In JavaScript, objects and functions are also variables.

In JavaScript, scope is a collection of accessible variables, objects, and functions.

JavaScript function scope: Scope is modified within the function.

JavaScript local scope

Variables are declared within a function, and the variables are local scope.

Local variables: can only be accessed inside the function.

Instance

// The carName variable cannot be called here

function myFunction() {
var carName = "Volvo";

// The carName variable can be called within the function

}

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Because Local variables only act within a function, so different functions can use variables with the same name.

Local variables are created when the function starts executing, and they will be automatically destroyed after the function is executed.

JavaScript global variables

Variables defined outside the function are global variables.

Global variables have global scope: all scripts and functions in the web page can be used.

Example

var carName = " Volvo";

// 此处可调用 carName 变量

function myFunction() {

    // 函数内可调用 carName 变量 

}

If a variable is not declared within a function (without using the var keyword), the variable is a global variable.

In the following example, carName is within the function, but is a global variable.

Instance

// 此处可调用 carName 变量

function myFunction() {
    carName = "Volvo";

    // 此处可调用 carName 变量

}

JavaScript variable lifecycle

JavaScript variable lifecycle is initialized when it is declared.

Local variables are destroyed after the function is executed.

Global variables are destroyed after the page is closed.

Function parameters

Function parameters only work within the function and are local variables.

Global variables in HTML

In HTML, global variables are window objects: all data variables belong to the window object.

Instance

//此处可使用 window.carName

function myFunction() {
    carName = "Volvo";
}

Did you know?

Your global variables, or functions, can override the variables or functions of the window object.

Local variables, including window objects, can override global variables and functions.

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