Questions
1 of 38
1What is the Iterator Protocol in JavaScript? What two things must an object implement to be a valid iterator?
2What does Symbol.asyncIterator do? How does for await...of use it?
3Can you chain or compose custom iterators? Implement a map and filter for a custom iterator without converting to an array.
4How does Array.from() use the iterator protocol? What's the difference between passing an iterable vs an array-like object?
5How would you implement an infinite iterator (e.g., an infinite counter)? How do you safely consume it?
6How do Map, Set, Array, and String expose their iterators? Are they the same object or different?
7Implement a take(n) utility that takes the first n values from any iterable — including infinite ones:
8What happens if you forget to return { done: true } in a custom iterator? What are the consequences?
9What is a 'lazy' iterator and why is it important for performance? Give a practical example.
10What are Iterator Helpers (.map(), .filter(), .take(), .drop() on iterators natively)? What stage are they at in the TC39 proposal pipeline?
11What does Symbol.iterator do? How would you make a plain object iterable from scratch?
12What is the difference between map.keys(), map.values(), and map.entries()? What do they return?
13What is the difference between an iterable and an iterator? Can something be both?
14How would you implement a reusable iterable (one that can be iterated multiple times)?
15What is the difference between a regular iterator and an async iterator? What protocol does an async iterator follow?
16What built-in JavaScript data structures are iterable by default? How does for...of work under the hood with them?
17How would you implement a readable stream as an async iterable in Node.js?
18How does destructuring and spread (...) use the iterator protocol internally?
19What does yield* do? How is it different from a regular yield?
20What is a generator function? How is it different from a regular function in terms of execution flow?
21Explain generator.return(value) and generator.throw(error). When would you use each in production?
22How would you use a generator to implement a tree traversal (DFS) without recursion stack concerns?
23Can you pass a value into a generator using .next(value)? How does that work and what is the practical use case?
24How do generators compose? Implement a pipeline of generator-based transformations (like RxJS operators but synchronous).
25When would you choose a generator over returning an array? What are the memory trade-offs?
26How do generators handle errors? How does .throw() interact with try/catch inside a generator?
27Explain what yield does. What does calling .next() return before and after a yield?
28What happens to a try/finally block inside a generator when .return() is called externally?
29What are the pitfalls of using for await...of with an async generator that makes network calls? How do you handle errors and cancellation?
30What are the debugging challenges with generators (e.g., in stack traces and async flows)? How do you mitigate them?
31How would you implement Promise-based async/await using generators and a runner function? (This is essentially how Babel transpiled async/await early on.)
32Are generators lazy? How do they differ from eager evaluation with arrays in terms of memory and performance?
33In a data pipeline processing millions of records, how would you use generators to avoid loading everything into memory?
34How would you use a generator to implement redux-saga-style side effect management? What makes generators a good fit for this?
35Implement an async generator that fetches paginated API data, yielding one page at a time:
36How can generators be used to implement coroutines or cooperative multitasking in JavaScript?
37What is the difference between yield and return inside a generator?
38What are the limitations of generators? What problems are they not a good fit for?
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