An iterable is an object that can be iterated over (it has a Symbol.iterator method). An iterator is an object that performs the iteration (it has a next method). An object can be both if it implements both methods.

An iterable is any object that implements the [Symbol.iterator]() method, which returns an iterator. Examples include Array, String, Map, and Set. An iterator is an object with a next() method that produces values. An object can be both iterable and iterator if it has [Symbol.iterator]() returning this and also a next() method, allowing it to be consumed by for...of while managing its own iteration state.

Difficulty: 5/10

Topics: iterable protocol, iterator protocol, for...of loop

Scenario Questions

0-2 years experience

1
You have an array of numbers and need to manually pull the first two items using its iterator. How would you get the iterator and retrieve those values?
2
If you try to use a plain object in a for...of loop, what error do you see and why?
3
Can a string be used directly in a for...of loop? Explain what makes that possible.

2-5 years experience

1
We refactored a utility to accept any iterable, but it now receives a custom object that implements Symbol.iterator and also defines a next method. The code breaks. Explain why and how you'd fix it.
2
During debugging you notice that spreading a Set into an array yields duplicate values. Walk through how iteration works for Set and why duplicates appear.
3
Implement a function that consumes an iterator once and returns a new iterable that can be iterated multiple times. What approach would you take?

5-8 years experience

1
Our data pipeline streams large JSON objects using a custom iterator to avoid loading everything into memory. Discuss the trade‑offs of exposing the iterator versus making the object iterable, and how you'd design the API for downstream services.
2
A library expects any iterable, but some callers pass an iterator that also implements Symbol.iterator returning itself. What are the performance and correctness implications when the library iterates more than once?
3
Design a wrapper that turns a one‑time iterator into a reusable iterable without materializing all items up front. What edge cases must you handle at scale?

8+ years experience

1
Our monorepo has legacy code that manually uses iterator protocols across modules. We're planning a migration to native for...of loops everywhere. Outline an architecture‑level plan, including handling objects that are both iterable and iterator and the impact on type definitions.
2
In a cross‑team data platform, some services expose streams as async iterables while others expose sync iterators. How would you create a unified abstraction, and what are the trade‑offs of forcing everything to be async iterable?
3
We currently expose a public API that returns custom iterator objects. Discuss the pros and cons of evolving it to return true iterables, and how to maintain backward compatibility for existing clients.

Follow-up Questions

❓What happens if you call next() after the iterator reports done?
❓Can you give an example of an object that is both iterable and iterator?
❓How does a for...of loop obtain the iterator under the hood?