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Ming-Shiuan Tsai
Ming-Shiuan Tsai

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JavaScript: Introduction to Scope (function scope, block scope)

What is Scope?

Scope determines the visibility or accessibility of a variable or other resource in the area of your code.

Global Scope

There's only one Global scope in the JavaScript document. The area outside all the functions is consider the global scope and the variables defined inside the global scope can be accessed and altered in any other scopes.

//global scope
var fruit = 'apple'
console.log(fruit);        //apple

function getFruit(){
    console.log(fruit);    //fruit is accessible here
}

getFruit();                //apple

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Local Scope

Variables declared inside the functions become Local to the function and are considered in the corresponding local scope. Every Functions has its own scope. Same variable can be used in different functions because they are bound to the respective functions and are not mutual visible.

//global scope
function foo1(){
    //local scope 1
    function foo2(){
        //local scope 2
    }
}

//global scope
function foo3(){
    //local scope 3
}

//global scope


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Local scope can be divided into function scope and block scope. The concept of block scope is introduced in ECMA script 6 (ES6) together with the new ways to declare variables -- const and let.

Function Scope

Whenever you declare a variable in a function, the variable is visible only within the function. You can't access it outside the function. var is the keyword to define variable for a function-scope accessibility.

function foo(){
    var fruit ='apple';
    console.log('inside function: ',fruit);
}

foo();                    //inside function: apple
console.log(fruit);       //error: fruit is not defined 


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Block Scope

A block scope is the area within if, switch conditions or for and while loops. Generally speaking, whenever you see {curly brackets}, it is a block. In ES6, const and let keywords allow developers to declare variables in the block scope, which means those variables exist only within the corresponding block.

function foo(){
    if(true){
        var fruit1 = 'apple';        //exist in function scope
        const fruit2 = 'banana';     //exist in block scope
        let fruit3 = 'strawberry';   //exist in block scope

    }
    console.log(fruit1);
    console.log(fruit2);
    console.log(fruit3);
}

foo();
//result:
//apple
//error: fruit2 is not defined
//error: fruit3 is not defined

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Lexical Scope

Another point to mention is the lexical scope. Lexical scope means the children scope have the access to the variables defined in the parent scope. The children functions are lexically bound to the execution context of their parents.

function foo1(){
    var fruit1 = 'apple';        
    const fruit2 = 'banana';     
    let fruit3 = 'strawberry';
    function foo2(){
        console.log(fruit1);
        console.log(fruit2);
        console.log(fruit3);
    }
    foo2();
}

foo1();

//result:
//apple
//banana
//strawberry

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For detailed comparison between var, let and const, take a look of JavaScript: var, let, const!

Discussion (24)

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tmsravan profile image
TMsravan

Nice explanation

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sashankramaraju profile image
Sashank Ramaraju

Very nice article on scopes !!!

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nguyentuan1696 profile image
Tuan Nguyen

Great clear explain !!!

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profgreatwonder profile image
Prof Great Wonder

Great article. Really helped me with understanding the block scope

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nvs16 profile image
N V S ABHISHEK

Nice article.

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caffro profile image
caffro

Thanks a mill, extremely helpful!

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mdsameershaikh profile image
mdsameershaikh

Hi if you not have written {curly brackets} for explaining block scope, I could not understand it.

thanyou verymych for expaining in such easy language.

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timrodz profile image
Juan Alejandro Morais

Great article - thanks for sharing! :)

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siddh profile image
Siddh

If we are not able to reassign the value in the variable that is defined with the const
then why it is changing his value every time, when the loop gets executed

const array = [1,2,3,4] ;

for (let index = 0; index < array.length; index++) {
const element = array[index];
console.log(element);
}

answer is

1
2
3
4

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devhamzaa profile image
Dev Hamza

you are storing the array[index] in element
you are not re-declaring but you are updating the value of element

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sanjarcode profile image
Muhammad Sanjar Afaq • Edited on

This is really helpful, embed this picture as a summary if you want.

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devhamzaa profile image
Dev Hamza

thanks dude i used it in my notes

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saroj8455 profile image
Saroj Padhan

very nice explanation

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mylifewithmyexperiment profile image
SHASHI JAISWAL

very very clear and vivid explanation. awesome

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gidoskales profile image
GidoSkales

Well Orchestrated ....

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mhamadsiro profile image
Mohamad-serhan

Fantastic, great job

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tuvudu profile image
Tuyen Vd.

thank for your explantation

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anhaltjan profile image
Jan Anhalt

This was very helpful, thank you very much! :)

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jfeijo profile image
Jo茫o Ant么nio Feij贸

Very cool explanation, straight to the point. I'm currently learning the scope subject on my bootcamp and you've made me understand it a lot better!

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saroj8455 profile image
Saroj Padhan

Nice explain of the scope

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vickcharles profile image
Vikler Charles

Very helpful article! :)

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seven07del profile image
Prince Djangmah

Awesome, simplified and tactical. Loved it.

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mahmoudabdulmuty profile image
Mahmoud Abdulmuty

Good Explanation .. thanks alot

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umeredava profile image
Umar Sanil

Good