ES2015's IterableIterator: Demystifying its Purpose

One thing that confuses me is when I implement the interface array, there are methods that return an IterableIterator. I understand that the IterableIterator extends from Iterator, which makes sense to me.

interface IterableIterator<T> extends Iterator<T> {
    [Symbol.iterator](): IterableIterator<T>;
}

However, I'm not sure what I should implement for Symbol.iterator. It seems like it also returns an IterableIterator (could this be recursion?) I am struggling with the concept of IterableIterator and would appreciate any insight or resources on this topic.

Answer №1

Iterator objects can be enhanced by making them Iterable, allowing for a variety of uses:

  • Utilizing them in a for..of loop
  • Spreading them into an array
  • Spreading them into a parameter list
  • Using them in APIs that accept iterables like
    • Array.from
    • new Set
    • new Map

An Iterator object that is not Iterable cannot be used in the ways mentioned above. The only option then is to call the .next() method repetitively, which can be inconvenient. Therefore, most Iterator objects produced by ECMAScript APIs are also made Iterable.

To convert an Iterator into an Iterable, the `[Symbol.iterator]` method should return this.

For further details, refer to:

  • “Iteration protocols” on MDN
  • “Implementing iterables” in Exploring ES6, which also discusses “Iterators that are iterable”

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