Object iterator property

index.js

const obj = {
  sub: {
    array: []
  },
  [Symbol.iterator]() {
    let index = 0;
    return {
      next: () => {
        // you could insert custom logic here, like stepping 3 by 3, filtering, etc.
        if ( index >= this.sub.array.length ) return { done: true }
        // use index value then increment it
        return { value: this.sub.array[index++], done: false }
      }
    }
  }
}

This is a nifty trick I was asked about in an interview.
I didn't know about it and failed the interview, but at least I learned something :-)

It's especially useful when you can't or don't want to modify a big data source and are mainly interested in one part of it.

You can then use it with the spread operator, or with a for...of loop to get the next() value out of it