🔰iterable

an object that can iterate over itself.

JS ⟩ iteration ⟩ iterable

⭐️ ES6 (2015)

an object that can make iterators. (like Sequence in Swift, must implement .makeIterator() method)

for (const value of iterable) { ... }    // for-of
[...iterable]                 // spread into array elements
f(...iterable)                // spread into function arguments
const [a, b, c] = iterable;   // destructuring iterable
Array.from(iterable)          // iterable -> array

🧨

(by default) object is not iterable❗

let obj = { x: 1, y: 2, z: 3 };
for (let value of obj) { }                   // ⛔ TypeError

// ⭐️ workarounds
for (let key of Object.keys(obj) { ... }
for (let key in obj) { ... }                 // for-in
for (let value of Object.values(obj) { ... }
for (let [key, value] of Object.entries(obj) { ... }

👉 for-in vs. for-of vs. in

arrays are iterated "live"❗️

⭐️

use cases：

for (const item of iterable) { ... } // ⭐️ for-of loop
[...iterable]                        // ⭐️ spread into array elements
f(...iterable)                       // ⭐️ spread into function arguments
let [a, b, c] = iterable             // ⭐️ iterable destructuring

// ⭐️ functios that accept iterables:
Array.from(iterable/array-like)
Promise.all(iterable)

iterables：

• Array, String, Set, Map, TypedArray,

• objects returned by generator functions.

• iterable examples

• NodeList, arguments object, ...

• array.keys(), map.entries(), ...

• str.matchAll()

must implement the make-iterator method named [Symbol.iterator].

some iterables only iterate once❗️

an iterable can be infinite. ( 👉 see： *integers())

🔴

• comforming to Iterable

  • make-iterator method - method named [Symbol.iterator].

  • Iterable+ext (extension) - extends iterables with custom methods.

  • obj.isIterable - check if an object is iterable.

• iterable types

  • String / Array / Set / Map

• 💾 use cases

  • for (const value of iterable) - for-of loop.

  • [...iterable] - spread into array elements.
  • f(...iterable) - spread into function arguments.
  • const [a, b, c] = iterable; - iterable destructuring.
  • Array.from(iterable) - convert to array.

• special iterable

  • only-iterate-once iterable

• defining iterables

  • make iterables

  • make iterator iterable

  • using class (with private properties)
• ✨ examples：

  • range() - half-open range of numbers.

  • *integers() - generate infinite iterable.

  • ClosedRange - closed range of integers.

  • *integers()

👥

• for loops

• Array

• array-likes and iterables are different things.

• destructuring assignment can be used with iterables.

🗺️

💈

• defining iterable (using generator function)

const iterable = {
    // ⭐️ "make-iterator" method
    *[Symbol.iterator]() {
        yield 1; yield 2; yield 3;
    },
};

more examples ...

• *integers() - generate non-negative integers.

• Sequence - sequence of numbers.

📗

• ExploringJS ⟩ 21. Iterables and Iterators ⭐️
• JS.info ⟩ iterables
• JavaScript: The Definitive Guide ⟩ Ch. 12

• 2ality ⟩ JavaScript needs more helper functions for iteration (map, filter, etc.)

📘

• for...of
• Array.from() - convert iterable / array-like to array.
• iterable protocol ( = Sequence protocol in Swift )
• Symbol.iterator ( = makeIterator in Swift )
• Built-in APIs accepting iterables
• Syntaxes expecting iterables

🗣

• What is the technical definition of a Javascript iterable and how do you test for it?