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META_SLIDE! loige loige.link/lets-iter 2

const array = ['foo', 'bar', 'baz'] for (const item of array) { console.log(item) } loige Output: foo bar baz 3

const str = 'foo' for (const item of str) { console.log(item) } loige Output: f o o 4

const set = new Set(['foo', 'bar', 'baz']) for (const item of set) { console.log(item) } loige Output: foo bar baz 5

const map = new Map([ ['foo', 'bar'], ['baz', 'qux'] ]) for (const item of map) { console.log(item) } loige Output: [ 'foo', 'bar' ] [ 'baz', 'qux' ] 6

const obj = { foo: 'bar', baz: 'qux' } for (const item of obj) { console.log(item) } loige Output: ⛔ Uncaught TypeError: obj is not iterable OMG `for ... of` does not work with plain objects! 😱 7

const obj = { foo: 'bar', baz: 'qux' } for (const item of Object.entries(obj)) { console.log(item) } loige Output: [ 'foo', 'bar' ] [ 'baz', 'qux' ] 8

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const array = ['foo', 'bar', 'baz'] console.log(...array) loige Output: foo bar baz spread syntax! 10

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loige Iterators Iterables Async Iterators Async Iterables Generator functions Async Generators 📸 THE BIGGER PICTURE 13

loige Knowing iteration protocols allows us: Understand JavaScript better Write more modern, interoperable and idiomatic code Be able to write our own custom iterables (even async) 😥 WHY SHOULD WE CARE? 14

WHO IS THIS GUY !? 👋 I'm Luciano ( 🍕🍝) Senior Architect @ fourTheorem (Dublin ) nodejsdp.link 📔 Co-Author of Node.js Design Patterns 👉 Let's connect! (blog) (twitter) (twitch) (github) loige.co @loige loige lmammino 15

ALWAYS RE-IMAGINING WE ARE A PIONEERING TECHNOLOGY CONSULTANCY FOCUSED ON AWS AND SERVERLESS | | Accelerated Serverless AI as a Service Platform Modernisation loige ✉ Reach out to us at 😇 We are always looking for talent: hello@fourTheorem.com fth.link/careers 16

We host a weekly podcast about AWS loige awsbites.com loige 17

📒 AGENDA Generators Iterator protocol Iterable protocol Async Iterator protocol Async Iterable protcol Real-life™ examples loige 18

LET'S CLONE 🐑🐏 loige loige.link/lets-iter-repo git clone https://github.com/lmammino/iteration-protocols-workshop.git cd iteration-protocols-workshop npm i 19

➡ GENERATORS loige 20

GENERATOR FN & OBJ loige function * myGenerator () { // generator body yield 'someValue' // ... do more stuff } const genObj = myGenerator() genObj.next() // -> { done: false, value: 'someValue' } 21

function * fruitGen () { yield ' 🍑 ' yield ' 🍉 ' yield ' 🍋 ' yield ' 🥭 ' } const fruitGenObj = fruitGen() console.log(fruitGenObj.next()) // { value: ' 🍑 ', done: false } console.log(fruitGenObj.next()) // { value: ' 🍉 ', done: false } console.log(fruitGenObj.next()) // { value: ' 🍋 ', done: false } console.log(fruitGenObj.next()) // { value: ' 🥭 ', done: false } console.log(fruitGenObj.next()) // { value: undefined, done: true } loige 22

function * fruitGen () { yield ' 🍑 ' yield ' 🍉 ' yield ' 🍋 ' yield ' 🥭 ' } const fruitGenObj = fruitGen() // generator objects are iterable! for (const fruit of fruitGenObj) { console.log(fruit) } // 🍑 // 🍉 // 🍋 // 🥭 loige 23

function * range (start, end) { for (let i = start; i < end; i++) { yield i } } // generators are lazy! for (const i of range(0, Number.MAX_VALUE)) { console.log(i) } const zeroToTen = [...range(0, 11)] loige 24

// generators can be "endless" function * cycle (values) { let current = 0 while (true) { yield values[current] current = (current + 1) % values.length } } for (const value of cycle(['even', 'odd'])) { console.log(value) } // even // odd // even // ... loige 25

📝 MINI-SUMMARY loige A generator function returns a generator object which is both an iterator and an iterable. A generator function uses `yield` to yield a value and pause its execution. The generator object is used to make progress on an instance of the generator (by calling `next()`). Generator functions are a great way to create custom iterable objects. Generator objects are lazy and they can be endless. 26

📝 EXERCISE(S) loige 02-generators/exercises/zip.js function * take (n, iterable) { // take at most n items from iterable and // yield them one by one (lazily) } 02-generators/exercises/zip.js function * zip (iterable1, iterable2) { // consume the two iterables until any of the 2 completes // yield a pair taking 1 item from the first and one from // the second at every step } 27

➡ ITERATORS & ITERABLES loige 28

ITERATOR OBJ loige An object that acts like a cursor to iterate over blocks of data sequentially 29

ITERABLE OBJ loige An object that contains data that can be iterated over sequentially 30

THE ITERATOR PROTOCOL An object is an iterator if it has a next() method. Every time you call it, it returns an object with the keys done (boolean) and value. loige 31

function createCountdown (from) { let nextVal = from return { next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } } } loige 32

const countdown = createCountdown(3) console.log(countdown.next()) // { done: false, value: 3 } console.log(countdown.next()) // { done: false, value: 2 } console.log(countdown.next()) // { done: false, value: 1 } console.log(countdown.next()) // { done: false, value: 0 } console.log(countdown.next()) // { done: true } loige 33

🔥 Generator Objects are iterators (and iterables)! loige 34

function * createCountdown (from) { for (let i = from; i >= 0; i--) { yield i } } loige 35

const countdown = createCountdown(3) console.log(countdown.next()) // { done: false, value: 3 } console.log(countdown.next()) // { done: false, value: 2 } console.log(countdown.next()) // { done: false, value: 1 } console.log(countdown.next()) // { done: false, value: 0 } console.log(countdown.next()) // { done: true, value: undefined } loige 36

THE ITERABLE PROTOCOL An object is iterable if it implements the Symbol.iterator method, a zero-argument function that returns an iterator. loige 37

function createCountdown (from) { let nextVal = from return { [Symbol.iterator]: () => ({ next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } }) } } loige 38

function createCountdown (from) { return { [Symbol.iterator]: function * () { for (let i = from; i >= 0; i--) { yield i } } } } loige 39

function * createCountdown () { for (let i = from; i >= 0; i--) { yield i } } loige 🔥 or just use generators! 40

const countdown = createCountdown(3) for (const value of countdown) { console.log(value) } // 3 // 2 // 1 // 0 loige 41

const iterableIterator = { next () { return { done: false, value: 'hello' } }, [Symbol.iterator] () { return this } } An object can be an iterable and an iterator at the same time! loige 42

📝 MINI-SUMMARY 1/2 loige An iterator is an object that allows us to traverse a collection The iterator protocol specifies that an object is an iterator if it has a `next()` method that returns an object with the shape `{done, value}`. `done` (a boolean) tells us if the iteration is completed `value` represents the value from the current iteration. You can write an iterator as an anonymous object (e.g. returned by a factory function), using classes or using generators. 43

📝 MINI-SUMMARY 2/2 loige The iterable protocol defines what's expected for a JavaScript object to be considered iterable. That is an object that holds a collection of data on which you can iterate on sequentially. An object is iterable if it implements a special method called `Symbol.iterator` which returns an iterator. (An object is iterable if you can get an iterator from it!) Generator functions produce objects that are iterable. We saw that generators produce objects that are also iterators. It is possible to have objects that are both iterator and iterable. The trick is to create the object as an iterator and to implement a `Symbol.iterator` that returns the object itself (`this`). 44

📝 EXERCISE(S) loige 04-iterable-protocol/exercises/binarytree.js class BinaryTree { // // ... // make this iterable! } 04-iterable-protocol/exercises/entries.js function entriesIterable (obj) { // Return an iterable that produce key/value pairs // from obj } 45

OK, very cool! But, so far this is all synchronous iteration. What about async? 🙄 loige 46

➡ ASYNC ITERATORS & ITERABLES loige 47

THE ASYNC ITERATOR PROTOCOL An object is an async iterator if it has a next() method. Every time you call it, it returns a promise that resolves to an object with the keys done (boolean) and value. loige 48

import { setTimeout } from 'node:timers/promises' function createAsyncCountdown (from, delay = 1000) { let nextVal = from return { async next () { await setTimeout(delay) if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } } } loige 49

const countdown = createAsyncCountdown(3) console.log(await countdown.next()) // { done: false, value: 3 } console.log(await countdown.next()) // { done: false, value: 2 } console.log(await countdown.next()) // { done: false, value: 1 } console.log(await countdown.next()) // { done: false, value: 0 } console.log(await countdown.next()) // { done: true } loige 50

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import { setTimeout } from 'node:timers/promises' // async generators "produce" async iterators! async function * createAsyncCountdown (from, delay = 1000) { for (let i = from; i >= 0; i--) { await setTimeout(delay) yield i } } loige 52

THE ASYNC ITERABLE PROTOCOL An object is an async iterable if it implements the `Symbol.asyncIterator` method, a zero-argument function that returns an async iterator. loige 53

import { setTimeout } from 'node:timers/promises' function createAsyncCountdown (from, delay = 1000) { return { [Symbol.asyncIterator]: async function * () { for (let i = from; i >= 0; i--) { await setTimeout(delay) yield i } } } } loige 54

HOT TIP 🔥 With async generators we can create objects that are both async iterators and async iterables! (We don't need to specify Symbol.asyncIterator explicitly!) loige 55

import { setTimeout } from 'node:timers/promises' // async generators "produce" async iterators // (and iterables!) async function * createAsyncCountdown (from, delay = 1000) { for (let i = from; i >= 0; i--) { await setTimeout(delay) yield i } } loige 56

const countdown = createAsyncCountdown(3) for await (const value of countdown) { console.log(value) } loige 57

📝 MINI-SUMMARY 1/2 loige Async iterators are the asynchronous counterpart of iterators. They are useful to iterate over data that becomes available asynchronously (e.g. coming from a database or a REST API). A good example is a paginated API, we could build an async iterator that gives a new page for every iteration. An object is an async iterator if it has a `next()` method which returns a `Promise` that resolves to an object with the shape: `{done, value}`. The main difference with the iterator protocol is that this time `next()` returns a promise. When we call next we need to make sure we `await` the returned promise. 58

📝 MINI-SUMMARY 2/2 loige The async iterable protocol defines what it means for an object to be an async iterable. Once you have an async iterable you can use the `for await ... of` syntax on it. An object is an async iterable if it has a special method called `Symbol.asyncIterator` that returns an async iterator. Async iterables are a great way to abstract paginated data that is available asynchronously or similar operations like pulling jobs from a remote queue. A small spoiler, async iterables can also be used with Node.js streams... 59

📝 EXERCISE loige 06-async-iterable-protocol/exercises/rickmorty.js function createCharactersPaginator () { // return an iterator that produces pages // with name of Rick and Morty characters // taken from the API // https://rickandmortyapi.com/api/character } // This is what we want to support 👇 const paginator = createCharactersPaginator() for await (const page of paginator) { console.log(page) } 60

➡ TIPS & ASYNC ITERATORS IN NODE.JS loige 61

function isIterable (obj) { return typeof obj[Symbol.iterator] === 'function' } const array = [1, 2, 3] console.log(array, isIterable(array)) // true const genericObj = { foo: 'bar' } console.log(isIterable(genericObj)) // false console.log(isIterable(Object.entries(genericObj))) // true const fakeIterable = { [Symbol.iterator] () { return 'notAnIterator' } } console.log(fakeIterable, isIterable(fakeIterable)) // true 😡 IS THIS OBJECT AN ITERABLE? loige 62

function isAsyncIterable (obj) { return typeof obj[Symbol.asyncIterator] === 'function' } IS THIS OBJECT AN ASYNC ITERABLE? loige Are there async iterable objects in Node.js core? 63

console.log( typeof process.stdin[Symbol.asyncIterator] === 'function' ) // true! let bytes = 0 for await (const chunk of process.stdin) { bytes += chunk.length } console.log(`${bytes} bytes read from stdin`) READABLE STREAMS! loige 64

import { createWriteStream } from 'node:fs' const dest = createWriteStream('data.bin') let bytes = 0 for await (const chunk of process.stdin) { // what if we are writing too much too fast?!! 😱 dest.write(chunk) bytes += chunk.length } dest.end() console.log(`${bytes} written into data.bin`) ⚠ BACKPRESSURE WARNING loige 65

import { createWriteStream } from 'node:fs' import { once } from 'node:events' const dest = createWriteStream('data.bin') let bytes = 0 for await (const chunk of process.stdin) { const canContinue = dest.write(chunk) bytes += chunk.length if (!canContinue) { // backpressure, now we stop and we need to wait for drain await once(dest, 'drain') // ok now it's safe to resume writing } } dest.end() console.log(`${bytes} written into data.bin`) loige ✅ handling backpressure like a pro! ... or you can use pipeline() 66

import { pipeline } from 'node:stream/promises' import {createReadStream, createWriteStream} from 'node:fs' await pipeline( createReadStream('lowercase.txt'), async function* (source) { for await (const chunk of source) { yield await processChunk(chunk) } }, createWriteStream('uppercase.txt') ) console.log('Pipeline succeeded.') loige pipeline() supports async iterables! 😱 67

import { on } from 'node:events' import glob from 'glob' // from npm const matcher = glob('**/*.js') for await (const [filePath] of on(matcher, 'match')) { console.log(filePath) } loige creates an async iterable that will yield every time the `match` event happens 68

import { on } from 'node:events' import glob from 'glob' // from npm const matcher = glob('**/*.js') for await (const [filePath] of on(matcher, 'match')) { console.log(filePath) } // ⚠ WE WILL NEVER GET HERE 👇 console.log('Done') loige This loop doesn't know when to stop! You could pass an AbortController here for more fine-grained control 69

📝 FINAL SUMMARY loige You can check if an object is an iterable by checking `typeof obj[Symbol.iterator] === 'function'` You can check if an object is an async iterable with `typeof obj[Symbol.asyncIterator] === 'function'` In both cases, there's no guarantee that the iterable protocol is implemented correctly (the function might not return an iterator 😥) Node.js Readable streams are also async iterable objects, so you could use `for await ... of` to consume the data in chunks If you do that and you end up writing data somewhere else, you'll need to handle backpressure yourself. It might be better to use `pipeline()` instead. You can convert Node.js event emitters to async iterable objects by using the `on` function from the module `events`. 70

Cover picture by on Back cover picture by on Camille Minouflet Unsplash Antonino Cicero Unsplash fourtheorem.com THANKS! 🙌 loige.link/lets-iter loige nodejsdp.link 71