What built-in JavaScript data structures are iterable by default? How does for...of work under the hood with them?

Under the hood, for...of obtains the iterator by calling the object's [Symbol.iterator]() method, then repeatedly calls the iterator's next() method, storing the value property in the loop variable and stopping when done is true. Example: const arr = [1,2,3]; for (const v of arr) { console.log(v); }