JavaScript Scope

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JavaScript Scope


Scope determines the accessibility (visibility) of variables.
JavaScript has 3 types of scope:

Block scope
Function scope
Global scope


Block Scope
Before ES6 (2015), JavaScript had only Global Scope and Function Scope.
ES6 introduced two important new JavaScript keywords: let and const.
These two keywords provide Block Scope in JavaScript.
Variables declared inside a { } block cannot be accessed
from outside the block:

Example

{
  let x = 2;
}
// x can NOT be used here

Variables declared with the var keyword can NOT have block scope.
Variables declared inside a { } block can be accessed from
outside the block.

Example

{
  var x = 2;
}
// x CAN be used here


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

Example

// code here can NOT use carNamefunction myFunction() {
  let carName = "Volvo";  // code here CAN use carName}
// code here can NOT use carName

Try it Yourself »


Local variables have Function Scope:
They can only be accessed from within the function.

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.

Function Scope
JavaScript has function scope: Each function creates a new scope.
Variables defined inside a function are not accessible (visible) from outside the
function.
Variables declared with var, let
and const are
quite similar when declared inside a function.
They all have Function Scope:


function myFunction() {
  var carName = "Volvo";  
// Function Scope
}




function myFunction() {
  let carName = "Volvo";   //
Function Scope
}




function myFunction() {
  const carName = "Volvo";   //
Function Scope
}



Global JavaScript Variables
A variable declared outside a function, becomes GLOBAL.

Example

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

Try it Yourself »


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


Global Scope
Variables declared Globally (outside any function) have
Global Scope.
Global variables can be accessed from anywhere in a JavaScript program.
Variables declared with var, let
and const are
quite similar when declared outside a block.
They all have Global Scope:


var x = 2;      
// Global scope



let x = 2;       //
Global scope



const x = 2;       //
Global scope


JavaScript Variables
In JavaScript, objects and functions are also variables.

Scope determines the accessibility of variables, objects, and functions from
different parts of the code.








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 a global variable carName, even if
the value is assigned inside a function.

Example

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


Strict Mode
All modern browsers support running JavaScript in "Strict Mode".
You will learn more about how to use strict mode in a later chapter of this tutorial.

In "Strict Mode", undeclared variables are not automatically global.


Global Variables in HTML
With JavaScript, the global scope is the JavaScript environment.
In HTML, the global scope is the window object.
Global variables defined with the var keyword belong to the window object:

Example

var carName = "Volvo";// code here
can use window.carName
Try it Yourself »

Global variables defined with the let keyword do not belong to the window object:

Example

let carName = "Volvo";
// code here can not use window.carName

Try it Yourself »


Warning

Do NOT create global variables unless you intend to.
Your global variables (or functions) can overwrite window variables (or
functions).Any function, including the window object, can overwrite your global
variables and functions.


The Lifetime of JavaScript Variables
The lifetime of a JavaScript variable starts when it is declared.
Function (local) variables are deleted when the function is completed.
In a web browser, global variables are deleted when you close the browser
window (or tab).

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












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Reference: https://www.w3schools.com/js/js_scope.asp