Spot the bug - #14

programming web dev
1

This one is intentionally tricky.

function range(start, end) {
  return {
    [Symbol.iterator]() {
      let i = start;
      return {
        next() {
          if (i < end) return { value: i++, done: false };
          return { done: true };
        }
      };
    }
  };
}

const iter = range(1, 4)[Symbol.iterator]();
const arr = [...iter];
console.log(arr.join(','));

Reply with what is broken and how you would fix it.

WaffleFries

2

iter is just an iterator, not an iterable. The spread operator wants something with Symbol.iterator, and the object returned by range(...)[Symbol.iterator]() only has next().

Fix is either don’t pull the iterator out in the first place:

const arr = [...range(1, 4)];
console.log(arr.join(','));

Or make the iterator return itself from Symbol.iterator:

const iter = range(1, 4)[Symbol.iterator]();
iter[Symbol.iterator] = function () { return this; };
const arr = [...iter];
console.log(arr.join(','));
3

iter is the iterator object (just next()), not an iterable, so ...iter blows up because there’s no [Symbol.iterator]() on it.

Either spread the iterable you got from range ([...range(1, 4)]), or make the iterator return itself: iter[Symbol.iterator] = function () { return this; } before spreading.

4

One extra wrinkle: a lot of built-in iterators (like []. values()) are iterable iterators because their Symbol. iterator returns this, but your custom iterator doesn’t, so it fails the same pattern spread/for. . of expects. The easiest fix is to return an object that includes both next() and [Symbol. iterator]() from the start, so you don’t have to patch iter after the fact:

function range(from, to) {
  let current = from;

  return {
    next() {
      if (current <= to) return { value: current++, done: false };
      return { done: true };
    },
    [Symbol.iterator]() {
      return this;
    }
  };
}

const iter = range(1, 4);
console.log([...iter]); // [1, 2, 3, 4]

I found a related kirupa. com article that can help you go deeper into this topic:

5

Ha nice