🔰iterator

JS⟩ iteration ⟩ iterator

⭐️ ES6 (2015)

object that can produce the next iteration result.

• has next() method that returns an iteration result. (similar to iterator objects conforming to IteratorProtocol in Swift)

⭐️ 重點

iterators only iterate once❗️

iterators：

• str.matchAll() - returns an iterator of matching results.

• objects returned by generator functions.

• array.keys(), map.entries(), ...

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

• next()

• return()cleanup

• throw()

are expected to return an iteration result object.

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

🔴 主題

• 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.

🗺️ 圖解

prototype chain of (built-in) iterators

📗 參考

• ExploringJS ⟩ 21. Iterables and Iterators ⭐️

• JavaScript: The Definitive Guide ⟩ 12.1 How Iterators Work

• Swift in Depth ⟩ Ch. 9 Iterators, sequences, and collections

📘 手冊

• function* declaration

• Iterators and generators

• iterator protocol ⟩

  • Array.prototype[@@iterator]()

👥 相關

• str.matchAll() returns an iterator of all matching results. (⭐️ ES2020)

🗣 討論

• Using map() on an iterator