# Understanding let vs var vs constant in Javascript

 All Javascript applications need to work with information in order to store information we make use of variables that are declared using *var*, *let*, *const* keywords.

> The major difference between the variable declared as var and the variables declared as let and const are var creates a function scoped variable whereas let and const create blocked scoped variables.

###### What is function scoped variable ?
*A function scoped variable is available anywhere declare inside the function*

**Note: if a var variable is not inside a function then it is globally scoped**

###### What is block scoped variable ?
*A block-scoped variable is only available inside the block it is declared in i.e if, while block *

```
if(true) {
   var  var_variable = "This is a variable declared with var keyword"
}
 console.log(var_variable) 
 //Output: "This is a variable declared with var keyword"
if(true) {
   let  let_variable = "This is a variable declared with let keyword"
}
 console.log(let_variable) 

 //Output: Uncaught ReferenceError: let_variable is not defined

```

> Another difference is the variable declare with var keyword can be completely redefined but let variable cannot be redefined 

```
var  var_variable = "This is a variable declared with var keyword"

var  var_variable = "var variable is redefined"
 
console.log(var_variable) 

 //Output: "var variable is redefined"

 let  let_variable = "This is a variable declared with let keyword";

 let  let_variable = "let variable is redefined" 
  
 console.log(let_variable)

 //Output: Uncaught SyntaxError: Identifier 'let_variable' has already been declared

```

> A variable declared with var can be used before it declared but in case of let or const it cannot be used.

**Note: if a variable is declared using const keyword it cannot be reassigned**

*The properties of objects declared with const keyword can be 
reassigned*

```
const userDetails = { id:121 };

userDetails.id = 125

console.log(userDetails) 

```

In the above example, the object `userDetails` is declared using the const keyword and its property `id` is reassigned.

Looking at all the above differences it usually recommended to use the let and const keyword over the var keyword.


That's All folks, Thank you.