Demystifying Async Programming in Javascript - Generator internals counts state machine

generator-internals-counts-state-machine.js

function counts(start) {
  let state = 0;
  let done = false;
  function go() {
    let result;
    switch (state) {
      case 0:
        result = start + 1;
        state = 1;
        break;
      case 1:
        result = start + 2;
        state = 2;
        break;
      case 2:
        result = start + 3;
        state = 3;
        break;
      case 3:
        result = start + 4;
        done = true;
        state = -1;
        break;
      default:
        break;
    }
    return {done: done, value: result};
  }
  return {
    next: go
  }
}
const counter = counts(0);
console.log(counter.next()); // {value: 1, done: false}
console.log(counter.next()); // {value: 2, done: false}
console.log(counter.next()); // {value: 3, done: false}
console.log(counter.next()); // {value: 4, done: true}
console.log(counter.next()); // {value: undefined, done: true}

I'm currently reading through the blog where this code snippet is presented and just have a quick question: Given that this is an example of how to build a generator using ES5, shouldn't the the lets be vars?