ES6 scoping with let and for

for_let_while_scoping.js

'use strict'

// For scoping is a bit surprising.
// Although i appears to be a persistent variable between iterations,
// actually there is a separately scoped i for each iteration!
for (let i = 0; i < 5; i++) {
  setTimeout(() => console.log(i), 10 * i)
}
// Prints: 0, 1, 2, 3, 4

// The same is not true for the while loop.  There is only one j.
let j = 0;
while (j < 5) {
  setTimeout(() => console.log(j), 100 + 10 * j)
  j++
}
// Prints: 5, 5, 5, 5, 5

// In fact it appears the for loop introduces a "middle" scope
// Look at the three scopes here:
let x = 5
for (let x = 0; x < 10; x++) {
  let x = 3;
  console.log(x);
}
// Pretty crazy right?

// By the way, this local scope means you can use const in these loop constructs:
for (const key in object) {
  console.log(key);
}
for (const item of array) {
  console.log(item);
}
// Although not in the classic C-style for we used earlier