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How do you write a basic generator function? Can you use a for...of loop directly on a generator's return value? How does yield pause and resume execution, and what does it return to the caller? How do you delegate from one generator to another using yield*? How do you implement an infinite sequence using a generator, and why is this safe? How do you use generator.throw() and generator.return() to control a running generator externally? How were generators historically used to write asynchronous code before async/await existed? How would you implement a custom Symbol.iterator for a tree data structure to enable depth-first traversal with for...of? What's the performance and memory advantage of a generator-based lazy pipeline over building intermediate arrays with map/filter?
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JavaScript › Iterators & Generators

What makes an object 'iterable' in JavaScript?

Beginner

Answer

An object is iterable if it implements the Symbol.iterator method, which returns an iterator object with a next() method. This is what powers for...of loops, spread syntax, and destructuring on that object.
const range = {
  [Symbol.iterator]() {
    let i = 1;
    return { next: () => i <= 3 ? { value: i++, done: false } : { value: undefined, done: true } };
  }
};
console.log([...range]); // [1, 2, 3]
Real-world example Making a custom collection class work seamlessly with for...of and spread, just like a built-in Array.

Common follow-ups: Which built-in types are iterable by default in JavaScript?

Types & Coercion
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