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Scope determines the accessibility (visibility) of variables
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:
{
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.
{
var x = 2;
}
// x CAN be used here
Variables declared within a JavaScript function, become LOCAL to the function.
// code here can NOT use carName
function myFunction() {
let carName = "Volvo";
// code here CAN use carName
}
// code here can NOT use carName
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.
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
}
A variable declared outside a function, becomes GLOBAL.
let carName = "Volvo";
// code here can use carName
function myFunction() {
// code here can also use carName
}
A global variable has Global Scope:
All scripts and functions on a web page can access it.
Variables declared Globally (outside any function) have Global Scope.
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
In JavaScript, objects and functions are also variables.
Scope determines the accessibility of variables, objects, and functions from different parts of the code.
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.
myFunction();
// code here can use carName
function myFunction() {
carName = "Volvo";
}
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.
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:
var carName = "Volvo";
// code here
can use window.carName
Global variables defined with the let
keyword do not belong to the window object:
let carName = "Volvo";
// code here can not use window.carName
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 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 (parameters) work as local variables inside functions.