let vs var in javascript

Question

I understand that let has block scope and var has functional scope. But I do not understand in this case, how using let will solve the problem

const arr = [1,2,3,4];
for (var i = 0; i < arr.length; i++) {
setTimeout(function() {
   console.log(arr[i]) 
}, 1000);
} // Prints undefined 5 times

const arr = [1,2,3,4];
for (let i = 0; i < arr.length; i++) {
setTimeout(function() {
   console.log(arr[i]) 
}, 1000);
} // Prints all the values correctly

Answer 1

First of all, the output will be four times and not five times(as mentioned in your comment). I pasted your code in Babel REPL and this is what I got,

"use strict";

var arr = [1, 2, 3, 4];

var _loop = function _loop(i) {
setTimeout(function () {
   console.log(arr[i]);
}, 1000);
};

for (var i = 0; i < arr.length; i++) {
_loop(i);
}

Do you see how let works internally now? :-)