iterator | web.dev

iterators：

iterators don't always run all the way to the end：

a for-of loop might be terminated with a break, return, or an exception.
when destructuring an iterable, the next() method (of an iterator) is only called enough times to obtain values for each variable.

All iterator protocol methods

are expected to return an iteration result object.

iterator for String works correctly with surrogate pairs. 👉 see： str.slice2()

features
- next() - returns an iteration result.
- return() - give iterator a chance to do cleanup actions.
- iterators only iterate once❗️ - iterators only iterate once❗️
- an iterator can be infinite.
use cases
- iterables use make-iterator method to make iterators.
defining iterators
- encapsulate iterator's state in a closure
extending iterators
- Iterator - extension of built-in iterators ⭐️
- make iterator iterable
- IteratorPrototype - prototype of all built-in iterators.
tyes of iterators
- iterable iterator - an iterator that is itself iterable.
- infinite iterator - iterator can be infinite.