Javascript Learning Var, Let and Const

 

With ES2016, two new ways to define variables were introduced with let and const. Before that you could use var to define variables.

Scope are mainly two type

1. Function Scope
2. Block Scope

1. Function Scope

function myFunction() {
            var firstName = 'Pradeep';
            console.log(firstName); 
            //Output : Pradeep 
            }
            console.log(firstName);
            //Output :  ReferenceError: firstName is not defined
        }

Using var, variables are function-scope because their visibility is limited to function. When you try to use it outside of the function, you’ll get an error.

2. Block Scope

if(true){
        var firstName = 'Pradeep';
        let test1 = 'Front end devloper';
        const test2 = 'and Ui Devloper';
        console.log(firstName);  // Pradeep;
        console.log(test1);    // front end devloper;
        console.log(test2);    // and Ui Devloper;
        }
        console.log(firstName);   // Pradeep;
        console.log(test1);   //ReferenceError: test1 is not defined;
        console.log(test2);   //ReferenceError: test2 is not defined

The visibility of firstName isn't limited by the if statement block. And test1 nad test2 are limited in to the block code.

What is var?

1.  The var statement declares a variable and can also optionally initialize its value.
2. var is function scoped when it is declared within function. That means that it is available and can be accessed only within that function
3. var are function scope because their visibility is limited to function. When you try to use outside the function you'll get an error.

 function myFun(){

var fname="Pradeep";

console.log(fname);  //Output : Pradeep 

}

 console.log(fname);  //Output :  ReferenceError: fname is not defined

or 

var x = 1;

if (x === 1) {

var x = 2;

console.log(x);

// expected output: 2

}

console.log(x);

// expected output: 2

What is let?

1. The let allows you to declear variables that are limited in scope to the perticular block scope. 
2.  block scope {} between the code 

    let x = 1;

if (x === 1) {

let x = 2;

console.log(x);

// expected output: 2

}

console.log(x);

// expected output: 1

 What is Const ?

Const are block scoped, much like variables defined using the let statement. the value of const cannot change through reassignment, and it can't be re-declared.

const number = 42;

try {

number = 99;

} catch (err) {

console.log(err);

// expected output: TypeError: invalid assignment to const `number'

// Note - error messages will vary depending on browser

}