Discussion on: Declaring JS Variables in 2019

[Ken Bellows]

Yeah, the difference seems to be that in a for ... of loop like this:

for (const thing of myThings) { /*...*/ }

you're essentially grabbing an iterator at the start of the loop, i.e. myThingsIter = myThings[Symbol.iterator](), then at the beginning of each loop running const thing = myThingsIter.next(). It seems to desugar to something like this:

const myThingsIter = myThings[Symbol.iterator]()
while (true){
  const {done, value: thing} = myThingsIter.next();
  if (done) break;

  /* ... */
}

You're always assigning a whole new value to the loop variable as opposed to modifying the loop variable as is typical in a classic for (let i=0; i<n; i++) loop.

Really the only reason you need a let in that loop is because you're reassigning the value with i++, equivalent to i = i+. In theory you can use a const if you're doing something super unusual, like using an object as your loop variable and modifying a property of it:

for (const o = {count: 0}; o.count < n; o.count++) {
    /* ... */
}

I don't know why anyone would ever want something like this, and there would undoubtedly be cleaner ways to write the code, but it works. You're never reassigning o, so no problem with it being a const.