Let, Var and Const in Javascript

Var

Var keyword has global Or Function scope, it can be re-declared inside its scope, variable should be followed by var.

function varGlobalScope(){
   var y=90;  // declare
   if(true){
       console.log(y); // 90
   }
   var y=89;//re-declare
   console.log(y) //89
}

Let

Let keyword has Block scope, It can’t be re-declared and can’t access out of block, variable should be followed by let.

function letRedeclare(){
    if(true){
      let y=25;
      console.log(y); // 25
      let y=90;  /* Uncaught SyntaxError: Identifier 'y' has already             been declared. **/      
    }
}
function letOutsideOfBlock(){
    if(true){
      let x=90;
       console.log(x)//90
     }
     console.log(x);// Uncaught ReferenceError: x is not defined
}

Const

Const behave same as let but you can’t change the value once you initialise, variable should be followed by Const.

function letOutsideOfBlock(){
    if(true){
          const x = 90;
          console.log(x) // 90
        try{
          x=78;
        }catch(error){
          console.log(error)// TypeError: Assignment to constant.               
        }
        console.log(x) // 90
    }
  console.log(x);// Uncaught ReferenceError: x is not defined
}