Iterators

An iterator is an object that allows us to lazily but synchronously consume multiple values. Iterators are not new to JavaScript, but there is a new standard forming at time of writing.

Iterators implement a next() method that returns an object that may have a value property, and may have a done property. Although the standard does not give this object a name, we will call it an iteration. If the iterator has produced the entirety of a sequence, the done property of the iteration will be true. Generator functions return iterators that expand on this basic definition. The value of a non-final iteration corresponds to a yield expression and the value of a done iteration corresponds to a return expression.

Iterators are an interface with many implementations. The canonical iterator yields the values from an array.

var iterator = iterate([1, 2, 3]);
var iteration = iterator.next();
expect(iteration.value).toBe(1);
iteration = iterator.next();
expect(iteration.value).toBe(2);
iteration = iterator.next();
expect(iteration.value).toBe(3);
iteration = iterator.next();
expect(iteration.done).toBe(true);

What distinguishes an iterator from an array is that it is lazy. An iterator does not necessarily end. We can have iterators for non-terminating sequences, like counting or the fibonacci sequence. The range function produces a sequence of values within an interval and separated by a stride.

function range(start, stop, step) {
    return {next: function () {
        var iteration;
        if (start < stop) {
            iteration = {value: start};
            start += step;
        } else {
            iteration = {done: true};
        }
        return iteration;
    }};
}

If the stop value of the range is Infinity, the iterator will have no end, and will never produce a done iteration. Unlike an array, an indefinite iterator consumes no more memory than an empty one.

var iterator = range(0, Infinity, 1);
expect(iterator.next().value).toBe(0);
expect(iterator.next().value).toBe(1);
expect(iterator.next().value).toBe(2);
// ...

The eager equivalent would produce an array, but would only work for bounded intervals since it must create an exhaustive collection in memory before returning.

function range(start, stop, step) {
    var result = [];
    while (start < stop) {
        result.push(start);
        start += step;
    }
    return result;
}

expect(range(0, 6, 2)).toEqual([0, 2, 4]);

Iterators may have alternate implementations of some methods familiar from arrays. For example, forEach would walk the iterator until it is exhausted. map would produce a new iterator of values passed through some transform, while filter would produce a new iterator of the values that pass a test. An iterator can support reduce, which would exhaust the iteration, but reduceRight would be less sensible since iterators only walk forward. Iterators may also have some methods that are unique to their character, like dropWhile and takeWhile.

We can save time and space by implementing pipelines with iterators instead of arrays. The following example can be interpreted as either eager or lazy, depending on whether range returns an array or an iterator. If we start with an array, map will create another array of 1000 values and filter will create another large array. If we start with an iterator, we will never construct an array of any size, instead percolating one value at a time as the reducer pulls them from the filter, as the filter pulls them from the mapping, and as the mapping pulls them from the range.

range(0, 1000, 1)
.map(function (n) {
    return n * 2;
})
.filter(function (n) {
    return n % 3 !== 0;
})
.reduce(function (a, b) {
    return a + b;
})