let and const

How let and const work

let and const are stored in a seprate memory space than var.

let a = 1;
const b = 2;
var c = 3;

{
  let a = 4;
  const b = 5;
  var c = 6;
  console.log(a); // 4
  console.log(b); // 5
  console.log(c); // 6
}

console.log(a); // 1
console.log(b); // 2
console.log(c); // 6

If you run the above code, you will see the following in dev tools:

There are 2 copies of a and b (in block and script scopes), but only one copy of c (in global scope). This is why a and b are not affected by the changes made in the block scope, but c is changed to 6.

warning

Even though let and var are stored in a separate memory space (let is stored in "script" scope i.e. DER while var is stored in global scope i.e. OER), you can't redeclare a let variable in the same scope using var:

let a = 1;
var a = 2; // SyntaxError: Identifier 'a' has already been declared

Doubts

References

Notes