Power or let (JS)

let.js

function letExample() {
  var a = [1,2,3,4,5],
    seq = Promise.resolve();

  for (let b of a) {
    seq = seq.then(() => new Promise(resolve => { setTimeout(()=> {console.log(b); resolve();},  100) }));
  }
}

function varExample() {
  var a = [1,2,3,4,5],
    seq = Promise.resolve();

  for (var b of a) {
    seq = seq.then(() => new Promise(resolve => { setTimeout(()=> {console.log(b); resolve();},  100) }));
  }
  //5, 5, 5, 5, 5  
}

letExample();   // -> 1, 2, 3, 4, 5

setTimeout(varExample, 1000); // -> 5, 5, 5, 5, 5