🔰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
✨ examples：
- range() - half-open range of numbers.
- *integers() - generate infinite iterable.
- ClosedRange - closed range of integers.
- *integers()

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.

Previousobj.isIterable Nextmake-iterator method

Last updated 2 years ago

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