const.js

a = 1;

console.error('global', typeof a); // undefined

const a; // const a is hoisted and set to undefined here first

a = 2; // has no affect

console.error('global after', typeof a); // still undefined

//const a = 1; // "TypeError: const 'a' has already been declared"

function whoo () {
  const a = 2; // new execution context so this works
  console.error('whoo', a); // 2
  ;(function inner () {
    a = 3; // silently ignored
    console.error('inner', a); // 2
  })()
}

whoo();

var.js

a = 1;
console.error('global', typeof a, a); // number, 1

var a; // var a is hoisted and set to undefined here first

a = 2;
console.error('global after', typeof a, a); // number, 2

var a = 1;
console.error('global after', typeof a, a); // number, 1

function whoo () {
  var a = 2; // new execution context
  console.error('whoo', a); // 2
  ;(function inner () {
    a = 3;
    console.error('inner', a); // 3
  })()
}

whoo();