JavaScript Scope(變數的範圍)

Scope is the set of variables you have access to.

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JavaScript Scope

In JavaScript, objects and functions, are also variables.

In JavaScript, scope is the set of variables, objects, and functions you have access to.

JavaScript has function scope: The scope changes inside functions.

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Local JavaScript Variables(內部作用)

Variables declared within a JavaScript function, become LOCAL to the function.

Local variables have local scope: They can only be accessed within the function.

// code here can not use carName
function myFunction() {
var carName = "Volvo";
// code here can use carName
}

 

Since local variables are only recognized inside their functions, variables with the same name can be used in different functions.

Local variables are created when a function starts, and deleted when the function is completed.

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Global JavaScript Variables

A variable declared outside a function, becomes GLOBAL.

A global variable has global scope: All scripts and functions on a web page can access it. 

var carName = " Volvo";
// code here can use carName
function myFunction() {
// code here can use carName
}

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Automatically Global(自動全局要小心！！)

If you assign a value to a variable that has not been declared, it will automatically become a GLOBAL variable.

This code example will declare carName as a global variable, even if it is executed inside a function.

// code here can use carName
function myFunction() {
    carName = "Volvo";
// code here can use carName
}

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The Lifetime of JavaScript Variables

• The lifetime of a JavaScript variable starts when it is declared.
• Local variables are deleted when the function is completed.
• Global variables are deleted when you close the page.
  

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Function Arguments

Function arguments (parameters) work as local variables inside functions.

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Global Variables in HTML

With JavaScript, the global scope is the complete JavaScript environment.

In HTML, the global scope is the window object: All global variables belong to the window object.全域變數是視窗物件等級，所以就成了視窗的一個屬性

// code here can use window.carName
function myFunction() {
    carName = "Volvo";
}

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Did You Know?
Your global variables, or functions, can overwrite window variables or functions.
Anyone, including the window object, can overwrite your global variables or functions.