TOPIC OVERVIEW

javascript/Iterator and Generator

Lazy, pausable functions that let JavaScript produce values on demand — one at a time.

10 QuestionsAdvancedHigh Priority ~12 min read

Most JavaScript functions run start to finish and hand back a single value. Generators don't. A function marked with function* can pause itself mid-execution at every yield, hand control back to whoever called it, and pick up exactly where it left off the next time you ask for a value. That's the core trick: instead of computing a whole sequence up front, a generator computes it lazily, one step at a time, only when something actually asks for the next item.

Iterators are the protocol underneath all of this. Any object that implements a next() method returning { value, done } is an iterator, and any object with a Symbol.iterator method that returns one is iterable — which is exactly what powers for...of, spread syntax, and destructuring under the hood. Generators are just the easiest way to build an object that satisfies that protocol without writing the state machine by hand.

What you'll walk away knowing

The exact shape of the iterator protocol: next(), { value, done }, and Symbol.iterator

How yield pauses execution and resumes with a value passed back into the generator

Why generators are a natural fit for infinite sequences and lazy data pipelines

How yield* delegates to another iterable, and how async generators extend the same protocol to for await...of

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

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