Difference between var, let and const in JAVASCRIPT;

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There are three different ways to declare variables in JavaScript. lets discuss the difference between them.

VAR   :

var a = 100;
var a = 100;
var a = 101;

a = 200;

console.log(a)

———

Output : 200 

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LET

let a = 100;
a  = 101


console.log(a)

Output : 101

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But for the following case it will give an error,

let a = 100;
let a = 100;
let a = 101;

console.log(a);

SyntaxError: Identifier ‘a’ has already been declared

Now lets consider the following piece of code.

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Using VAR

function varTest() {
  var x = 1;
  {
    var x = 2;  // same variable!
    console.log(x);  // 2
  }
  console.log(x);  // 2
}

OUTPUT : 

2
2

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USING LET

function letTest() {
  let x = 1;
  {
    let x = 2;  // different variable
    console.log(x);  // 2
  }
  console.log(x);  // 1
}

OUTPUT :

2
1

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————————————————————————————–

CONST

const a = 100;
const a = 100;
const a = 101;

a = 101;

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In any of the above cases it will show some error

Assignment to constant variable

• So var works in a global scope.

• Let works in a local scope. Outside of the scope its value automatically gets de-assigned.

• Const is something similar to let but , in any case we can not the value. It’s more stricter than let. Also for const we have to initialize the moment we declare it.

 If you run this following code, it will throw an error if we don’t initialize the const variable

const a;
a = 100;

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 You will get an error.

There is another significant difference between var and let.

Because var works in the global scope so if any variable is being accessed     before initializing it, it will be hoisted and will be assigned an “undefined” value.

console.log(b);
 
var b;

If you run the above code , you will get an “undefined” value.

But in-case of let it won’t be accessible and it will give an error.

console.log(b);
 
let b;

Cannot access ‘b’ before initialization

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