Scope is the set of variables you have access to.
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.
Local JavaScript Variables
Variables declared within a JavaScript function, become LOCAL to the function.
Local variables have local scope: They can only be accessed within the function.
Example
// code here can not use carName
function myFunction() {
var carName = "Volvo";
// code here can use carName
}
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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.
JavaScript Global 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.
Example
var carName = " Volvo";
// code here can use carName
function myFunction() {
// code here can use carName
}
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Automatically Global
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.
Example
// code here can use carName
function myFunction() {
carName = "Volvo";
// code here can use carName
}
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Do NOT create global variables unless you intend to.
In "Strict Mode" automatically global variables will fail.
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.
Function Arguments
Function arguments (parameters) work as local variables inside functions.
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.
Example
// 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).
Any function, including the window object, can overwrite your global
variables and functions.