💾Iterator

extending built-in iterable iterators with methods map(), filter() ...

JS⟩ iteration ⟩ iterator ⟩ Iterator (extension)

(deprecated , use Iterable+ext instead)

extending built-in iterable iterators with methods map(), filter() ...

prototype chain of (built-in) iterators

💾 程式

• replit ⟩ Iterator (old), Iterator.prototype (new)

// --------------------
//     ⭐ Iterator
// --------------------
// 🧨 雷區：
// • must use function declaration. (can't use class declaration)
// • the prototype of a class is unwritable❗
function Iterator() {}            // function declaration

// ⭐ reassign/overwrite prototype❗️ 
// • prototype of all built-in (iterable) iterators
Iterator.prototype = Object.getPrototypeOf(     // IteratorPrototype
    Object.getPrototypeOf(                      // ArrayIteratorPrototype
        [][Symbol.iterator]()                   // Array iterator
    )
);

// ⭐ extending `Iterator.prototype`
// ---------------------------------
// - 🔸 .filter()     -> generator
// - 🔸 .map()        -> generator
// - 🔸 .take()       -> generator
//
// - 🔸 .every()      -> boolean
// - 🔸 .some()       -> boolean
// - 🔸 .toArray()    -> array
// - 🔸 .forEach()    -> (returns `undefined`)

// 🔸 .filter()
Iterator.prototype.filter = function*(condition) {
    for (const value of this) {
        if (condition(value)) yield value;
    }
};

// 🔸 .map()
Iterator.prototype.map = function*(f) {
    for (const value of this) { yield f(value) }
};

// 🔸 .take()
// turn "infinite" iterator into "finite"
Iterator.prototype.take = function*(n) {
    for (const value of this) {
        if (n > 0) {
            n -= 1;
            yield value;
        } else { return }
    }
};

// 🔸 .forEach()
Iterator.prototype.forEach = function(handle) {
    for (const value of this) { handle(value) }
};

// 🔸 .every()
Iterator.prototype.every = function(condition) {
    for (const value of this) { if (!condition(value)) return false }
    return true;
};

// 🔸 .some()
Iterator.prototype.some = function(condition) {
    for (const value of this) { if (condition(value)) return true }
    return false;
};

// 🔸 .toArray()
Iterator.prototype.toArray = function() {
    return [...this];
};

// ⭐️ export
module.exports = Iterator;

💈範例

• 💾 replit ⟩ Iterator, require： *integers()

// ⭐ import
const Iterator = require('./Iterator.js');    // extend iterators
const integers = require('./integers.js');    // infinite generator

// ✅ test method chaining: 
//   Note: 
//   - each method returns a different generator
//     (except for .forEach, which returns `undefined`)
integers()                         // 👔 generator: 0, 1, 2, ...
  .filter(x => x > 0)              // 👔 generator: 1, 2, 3, ...
  .map(x => x * 2)                 // 👔 generator: 2, 4, 6, ...
  .take(4)                         // 👔 generator: 2, 4, 6, 8
  .forEach(x => console.log(x))    // 👔 console: 2, 4, 6, 8
;

// ⭐ prototype chain (of a String iterator)
let it = 'abc'[Symbol.iterator]();            // iterator of String
let proto1 = Object.getPrototypeOf(it);       // StringIteratorPrototype
let proto2 = Object.getPrototypeOf(proto1);   // IteratorPrototype
let proto3 = Object.getPrototypeOf(proto2);   // Object.prototype
let proto4 = Object.getPrototypeOf(proto3);   // null

// log
[
    // ⭐ iterator only iterates once❗
    it.toArray(),                  // [ 'a', 'b', 'c' ]
    it.toArray(),                  // []     ⭐ `it` is "exhausted"❗
    
    // ✅ test prototype chain
    it,                            // String iterator
    it instanceof Iterator,        // true
    proto1,                        // StringIteratorPrototype
    proto2 === Iterator.prototype, // true
    proto3 === Object.prototype,   // true
    proto4 === null,               // true

    // ✅ test .take()
    integers().take(5).toArray(),  // [ 0, 1, 2, 3, 4 ]

    // ✅ test .every(), .some()
    integers()                // 0, 1, 2, ...
        .take(5)              // 0, 1, 2, 3, 4
        .every(x => x >= 0),  // true

    integers()                
        .take(5)              
        .every(x => x < 3),   // false

    integers()                
        .take(5)              
        .some(x => x < 3),    // true

].forEach(x => { console.log(x) });

⬇️ 應用

• extended by Iterator

  • *integers()

  • *fibonacci()

  • *zip() - zip a list of iterables into a single generator function.

  • *sequence()

  • *interleave()

• require IteratorPrototype

  • printPrototypeChain() - print prototype chain of an object.

👥 相關

• class ⟩ 🧨 雷區 - can't overwrite class's prototype.

📗 參考

• 2ality ⟩ introducing a superclass for iterators
• JavaScript: The Definitive Guide ⟩ 12.2 Implementing Iterable Objects

📘 手冊

• Set.prototype.values() - returns an iterator.
• Object.values() - returns an array❗️

🗣 討論

• Javascript: Overwriting function's prototype - bad practice?