let.js

"use strict"

var a = 'foo';

function foo() {
  console.log(a);
  var a = 'bar';
  console.log(a);
};

foo();

// LOG:
// undefined
// bar


"use strict"

var a = 'foo';

function foo() {
  console.log(a);
  let a = 'bar';
  console.log(a);
};

foo();

// LOG:
//   console.log(a);
//               ^
// ReferenceError: a is not defined