Javascript var let const

Ref.: Var, Let, and Const – What’s the Difference?

var is globally scoped or function scoped.

var can be re-declared

Hoisting of var

They are all hoisted to the top of their scope.

console.log (greeter);
var greeter = "say hello"

interpreted as:

var greeter;
console.log (greeter);
greeter = "say hello"

var

var greeter = "hey hi";
var times = 4;

if (time > 3) {
  var greeter = "say Hello instead";
}

console.log(greeter); // "say Hello instead"

While this is not a problem if you knowingly want greeter to be redefined, it becomes a problem when you do not realize that a variable greeter has already been defined before.

let and const are block scoped

let and const cannot be updated

Hoisting of let

Unlike var initialized as undefined, the let keyword is not initialized. You’ll get a Reference Error before declaration.

const cannot be updated