Enough JavaScript to get you Started : 16 var vs let vs const

Before we start

Before we start this article i would like to clarify some technical jargons for you

Scope : Scope is nothing but a code block where the variable is accessible for usage

Global Scope : Global Scope means variable is declared globally (not in some condition or function) hence it can be used any where throughout the execution of program

Local/ Functional Scope : this simply means that when we declare a variable on function level or somewhere locally in code block then it's not accessible outside of that particular scope (imagine variables declared in functions , loops ,conditionals...)

Block Scope : Blocks are nothing but piece of code written inside any curly braces {...} [ex. if block , or function's block]

Var

Var is the oldest way of declaring variable

Var can be globally and functionally scoped

If we Declare var inside function it becomes functionally scoped if we declare it outside function it becomes globally scoped and is available anywhere in program

can be redeclared or updated

Example of Scope

var a = 10; // global variablefunction fun(){    // functional scoped variable    var b = 20;    console.log(a);    console.log(b);}fun();console.log(a);console.log(b);

output

-> inside function10 20 -> outside function10 uncaught reference : b is not defined  

Notice that functions can access both global and functional variables.

Example of Re-declaration

// variables with var can be re-decalred var a = 10;var a = 20;// you won't get any error

Example of updatable variables

// variables with var can be updated var a =10;a=20;// you won't get any error

Problems with var

Redefining variables won't throw any errors which means it's tricky to remember which variable is already there and which variable is new.

Let

Let is the modern way of declaring variables introduces in ES2015

Let is now recommended way of declaring variables

Let is block scoped

Let can be updated but not redeclared

Example of Declaration

// let can be updated let a = 30; a = 40; // but not redeclared let b = 40; let b = 90 ;// error : Identifier 'b' has already been declared

Example of block scope

let sayHi = "hi";if(sayHi === "hi"){    let newMsg = "how are you?";   console.log(sayHi); // outputs "hi"}console.log(sayHi); console.log(newMsg);  // 'newMsg is not defined'

Const

Variables declared with const remains same throughout the execution

variables declared with const are not redeclarable or updatable

if const variables declared outside any block they becomes global scoped , but if they are declared within block they becomes block scoped

values can not be changes or new values can not be assigned to const variables

Example:

const sayHi = "hi";const sayHi = "hello";  // will throw error//and ...const msg = "buy bread from market";msg = "new msg here";  // error:  assignment to constant variable.

But...

const numbers = [1,2,3];number[0] = 4;  // works fine// but...numbers = [5,6,7];  // won't work

the first case will work because we're within the rules , we're not redeclaring the const variable nor we're updating it. but we're mutating it.

Summary

var: Global/function scoped depending on declaration undefined when accessing a variable before it's declared. can be redeclared and updated.


let: block scoped. can be updated but we can't redeclare.


const: block scoped. can not be reassigned nor we can redeclare.

Let me know in comment section if you have any doubt or feedback. it's always worth to give time to the thriving developer community :)

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