In ES6, there is iterator.next(); is there any way to supply iterator.previous()?

Question

Going through the full ES6 Compatibility table. Just got on to Set().

const set = new Set();
set.add('foo');
set.add('baz');

const iterator = set.values();
iterator.next(); // { value: "foo", done: false }
iterator.next(); // { value: "baz", done: false }

Is it possible to write a method similar to iterator.next(), but it iterates backwards instead of forwards (i.e. iterator.previous())?

Answer 1

The values() returns an iterator object and it is not possible to iterate them backwards, because JavaScript iterator objects could be infinite. For example, consider this

function * Counter() {
    "use strict";
    var i = 0;
    while (1) {
        yield i++;
    }
}

Now, you can create an iterator with Counter() and that will never end. So going backwards is not an option with iterators, generally.

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If you badly need something like backIterator, then you have to maintain the values produced by the iterator and then move back and forth based on the next calls.