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
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
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
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
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
For detailed comparison between var, let and const, take a look of JavaScript: var, let, const!
Top comments (35)
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.
Nice article.
Great article. Really helped me with understanding the block scope
Great clear explain !!!
Great article - thanks for sharing! :)
Nice explanation
Very nice article on scopes !!!
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
you are storing the array[index] in element
you are not re-declaring but you are updating the value of element
Thank you @mingt .
I think there is a misinformation In "Lexical Scope" topic that you define it: "children scope have the access to the variables defined in the parent scope."
I think that definition is for "Scope Chain" and the "Lexical Scope" related to which type of referencing to parent in Scope Chain does JS (Lexical or Dynamic).
Very helpful article! :)