What Is The Difference Between Let And Var In JavaScript

 

Hey, learner Today we’ll explore the Let and Var. If you’re a beginner in this field, then follow me because I’m going to explain it from scratch. 

So, Let and Var are keywords in Javascript with the help of we declare variables. Before moving ahead let’s discuss what is variable for a short introduction to make the topic clear for beginners. I know most of you have heard this name earlier. If you haven’t, so don’t worry we’ll cover it up here. 

Variables

Variables are containers for storing data (values). In data, it can be number, name, boolean, string, object, etc.

In this below example, p, quantity, name, response are declared with the var keyword:

var p = 15;
var quantity = 5;
var name = "Codewithrandom";
var response = 'Yes I am!';

From the above example, you can expect:

We have some rules while declaring a JavaScript variable.  All Javascript variables must be a unique name.

var var_name;
var p;

After the declaration, if we don’t assign any value or data to the variable then (technically it carries the value of undefined).

var p; // undefined

If you will run it then it prints the undefined value on the console.

p = 15;
var milkName = "DMS";

We can also assign a value to the variable at the time of declaration

Let’s have a look glimpse of var.

var person = "Rohan", profession = "programmer", Home = "USA";
console.log(profession);
console.log(person);
console.log(Home);
// it will print one by one each value which is collectively declare with var.

2.  For mathematical expression.

var x = 5 + 20 + 1*3;
console.log(x); // output 28

3. Variable with re-declaration.

var milkName = "MDS";
var milkName;
console.log(milkName);

Var as a Local variable.

myfunction();
function myfunction() {
var routeName = "Sinifer 28"; // local variabl
console.log(typeof(routeName) + " " + routeName);
}
console.log(typeof(routeName));
//string Sinifer 28
//undefined

The above example illustrates the use of a local variable. However, that a statement outside of the function can’t refer to the variable named routeName. So, it is showing an error of undefined it is so because it has local scope.

Var as a Global variable and Scope.

1: var routeName = "Sinifer 28"; //global variabl
2: myfunction();
3: function myfunction() {
4: console.log(typeof(routeName) + " "+"I have to go on this way" + routeName);
5: }
6: console.log(typeof(routeName)+ " "+ "I have to go on this way" + routeName);
7: //string I have to on this waySinifer 28
8: //string I have to on this waySinifer 28

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let x = "box1";
let x = 13;
console.log(x);
//Identifier 'x' has already been declared

• Variables defined with let must be Declared before use

• For declaring single and multi-variable we can use it.

 let x=5, y=4, z=9;
console.log(x);
console.log(y);
console.log(z);

{
let id="MIB";
console.log(id);
}
console.log(id);
//MIB
//console.log(id);
^
//ReferenceError: id is not defined

Global Scope

 let num=101;
console.log(num);
function fun(){
console.log(num);
}
fun(); // calling the function
//101
//101

Function Scope

function add(){
let num=20;
console.log(num);
}
add(); // calling the function
console.log(num);
//20
//console.log(num);
^
//ReferenceError: num is not defined

Thanks For Reading

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written by: codewave