Did you know JavaScript has iterators? DublinJS

META_SLIDE!
loige
loige.link/iterate
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String[] transactions = {
"paid 20",
"received 10",
"paid 5",
"received 15",
"paid 10",
"received 12"
};
var total = Stream.of(transactions).mapToInt(transaction -> {
var parts = transaction.split(" ");
int amount = Integer.decode(parts[1]);
if (Objects.equals(parts[0], "paid")) {
amount = -amount;
}
return amount;
}).sum();
if (total >= 0) {
System.out.println("Life is good :)");
} else {
System.out.println("You are broke :(");
}
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replit.com/@lmammino/r-u-broke-java
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"paid 20",
"received 10",
"paid 5",
"received 15",
"paid 10",
"received 12"
};
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amount = -amount;
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}
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return amount;
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}).sum();
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if (total >= 0) {
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} else {
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}
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"paid 20",
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"received 10",
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"paid 5",
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"received 15",
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"paid 10",
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"received 12"
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};
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amount = -amount;
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}
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return amount;
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}).sum();
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if (total >= 0) {
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} else {
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}
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amount = -amount;
}
return amount;
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"paid 20",
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"received 10",
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"paid 5",
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"received 15",
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"paid 10",
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"received 12"
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};
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}).sum();
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if (total >= 0) {
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} else {
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}
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}).sum();
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"paid 20",
2
"received 10",
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"paid 5",
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"received 15",
5
"paid 10",
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"received 12"
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};
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amount = -amount;
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}
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return amount;
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if (total >= 0) {
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} else {
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}
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if (total >= 0) {
} else {
}
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"paid 20",
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"received 10",
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"paid 5",
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"received 15",
5
"paid 10",
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"received 12"
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};
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amount = -amount;
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}
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return amount;
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}).sum();
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"paid 20",
"received 10",
"paid 5",
"received 15",
"paid 10",
"received 12"
};
amount = -amount;
}
return amount;
}).sum();
if (total >= 0) {
} else {
}
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replit.com/@lmammino/r-u-broke-java
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transactions = [
"paid 20",
"received 10",
"paid 5",
"received 15",
"paid 10",
"received 12"
];
def get_amount(transaction):
type, amount = transaction.split(' ')
amount = int(amount)
if type == 'paid':
return -amount
return amount
*_, total = accumulate(
map(get_amount, transactions)
)
if total >= 0:
print("Life is good :)")
else:
print("You are broke :(")
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replit.com/@lmammino/r-u-broke-py
5

const transactions = [
"paid 20",
"received 10",
"paid 5",
"received 15",
"paid 10",
"received 12"
]
const total = Iterator.from(transactions)
.map(transaction => {
let [action, amount] = transaction.split(" ")
amount = Number.parseInt(amount)
if (action === "paid") {
return -amount
} else {
return amount
}
})
.reduce((acc,curr) => acc + curr)
if (total >= 0) {
console.log("Life is good :)")
} else {
console.log("You are broke :(");
}
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replit.com/@lmammino/r-u-broke-js
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"paid 20",
"received 10",
"paid 5",
"received 15",
"paid 10",
"received 12"
]
return -amount
} else {
return amount
}
})
if (total >= 0) {
} else {
}
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"paid 20",
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"received 10",
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"paid 5",
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"received 15",
5
"paid 10",
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"received 12"
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]
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return -amount
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} else {
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return amount
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}
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})
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if (total >= 0) {
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} else {
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}
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"paid 20",
2
"received 10",
3
"paid 5",
4
"received 15",
5
"paid 10",
6
"received 12"
7
]
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return -amount
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} else {
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return amount
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}
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})
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if (total >= 0) {
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} else {
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}
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"paid 20",
2
"received 10",
3
"paid 5",
4
"received 15",
5
"paid 10",
6
"received 12"
7
]
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return -amount
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} else {
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return amount
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}
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})
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if (total >= 0) {
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} else {
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}
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"paid 20",
"received 10",
"paid 5",
"received 15",
"paid 10",
"received 12"
]
return -amount
} else {
return amount
}
})
if (total >= 0) {
} else {
}
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loige
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"paid 20",
"received 10",
"paid 5",
"received 15",
"paid 10",
"received 12"
]
return -amount
} else {
return amount
}
})
if (total >= 0) {
} else {
}
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loige
This is not JavaScript, it's FutureJavaScript™
github.com/tc39/proposal-iterator-helpers
9

import Iterator from 'core-js-pure/actual/iterator/index.js'
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"paid 20",
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"received 10",
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"paid 5",
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"received 15",
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"paid 10",
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"received 12"
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]
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return -amount
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} else {
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return amount
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}
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})
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if (total >= 0) {
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} else {
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27 loige
But if you want the future, today...
npm i --save core-js-pure
10

loige
ITERATORS ARE LAZY!
You can consume the collection 1 item at the time
You don't need to keep all the items in memory
Great for large datasets
You can even have endless iterators!
12

loige
The concept of Iterators exists already in JavaScript...
🗞GOOD NEWS FROM THE WORLD...
... since ES2015!
13

WAIT, WHO THE HECK 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
14

ALWAYS RE-IMAGINING
WE ARE A PIONEERING TECHNOLOGY CONSULTANCY FOCUSED
ON CLOUD, AWS & SERVERLESS
loige
😇We are always looking for talent: fth.link/careers
We can help with:
Cloud Migrations
Training & Cloud enablement
Building serverless applications
Cutting cloud costs
15

loige
podcast
awsbites.com
IF YOU LIKE AWS...
16

loige
🥤
LET'S HAVE A QUICK REFRESHER
ON SOME JS "ITERATION" CONCEPTS...
17

const array = ['foo', 'bar', 'baz']
for (const item of array) {
console.log(item)
}
loige
Output:
foo
bar
baz
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const str = 'foo'
for (const item of str) {
console.log(item)
}
loige
Output:
f
o
o
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const set = new Set(['foo', 'bar', 'baz'])
for (const item of set) {
console.log(item)
}
loige
Output:
foo
bar
baz
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const map = new Map([
['foo', 'bar'], ['baz', 'qux']
])
for (const item of map) {
console.log(item)
}
loige
Output:
[ 'foo', 'bar' ]
[ 'baz', 'qux' ]
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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! 😱
22

const obj = {
foo: 'bar',
baz: 'qux'
}
for (const item of Object.entries(obj)) {
console.log(item)
}
loige
Output:
[ 'foo', 'bar' ]
[ 'baz', 'qux' ]
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const array = ['foo', 'bar', 'baz']
console.log(...array)
loige
Output:
foo bar baz
spread syntax!
25

📒AGENDA
Generators
Iterator protocol
Iterable protocol
Async Iterator protocol
Async Iterable protcol
Real-life™ examples
loige 28

GENERATOR FN & OBJ
loige
function * myGenerator () {
// generator body
yield 'someValue'
// ... do more stuff
}
const genObj = myGenerator()
genObj.next() // -> { done: false, value: 'someValue' }
30

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 31

function * fruitGen () {
yield '🍑'
yield '🍉'
yield '🍋'
yield '🥭'
}
const fruitGenObj = fruitGen()
// generator objects are iterable!
for (const fruit of fruitGenObj) {
console.log(fruit)
}
// 🍑
// 🍉
// 🍋
// 🥭
loige 32

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 33

// 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 34

📝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.
35

➡ITERATORS & ITERABLES
loige 36

ITERATOR OBJ
loige
An object that acts like a cursor to iterate over blocks of data sequentially
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ITERABLE OBJ
loige
An object that contains data that can be iterated over sequentially
38

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 39

function createCountdown (from) {
let nextVal = from
return {
next () {
if (nextVal < 0) {
return { done: true }
}
return {
done: false,
value: nextVal--
}
}
}
} loige 40

const countdown = createCountdown(3)
console.log(countdown.next())
// { done: false, value: 3 }
// { done: true } loige 41

🔥
Generator Objects are iterators (and iterables)!
loige 42

function * createCountdown (from) {
for (let i = from; i >= 0; i--) {
yield i
}
}
loige 43

// { done: true, value: undefined } loige 44

THE ITERABLE PROTOCOL
An object is iterable if it implements the
Symbol.iterator method, a zero-argument function
that returns an iterator.
loige 45

let nextVal = from
return {
[Symbol.iterator]: () => ({
next () {
if (nextVal < 0) {
}
return { done: false, value: nextVal-- }
}
})
}
}
loige 46

return {
[Symbol.iterator]: function * () {
for (let i = from; i >= 0; i--) {
yield i
}
}
}
}
loige 47

function * createCountdown () {
for (let i = from; i >= 0; i--) {
yield i
}
}
loige
🔥or just use generators!
48

for (const value of countdown) {
console.log(value)
}
// 3
// 2
// 1
// 0
loige 49

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 50

📝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.
51

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`).
52

OK, very cool!
But, so far this is all synchronous iteration.
What about async? 🙄
loige 53

➡ASYNC ITERATORS & ITERABLES
loige 54

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 55

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: false, value: nextVal-- }
}
}
} loige 56

const countdown = createAsyncCountdown(3)
console.log(await countdown.next())
// { done: true } loige 57

// async generators "produce" async iterators!
async function * createAsyncCountdown (from, delay = 1000) {
for (let i = from; i >= 0; i--) {
yield i
}
}
loige 59

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 60

function createAsyncCountdown (from, delay = 1000) {
return {
[Symbol.asyncIterator]: async function * () {
for (let i = from; i >= 0; i--) {
yield i
}
}
}
}
loige 61

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 62

// async generators "produce" async iterators
// (and iterables!)
async function * createAsyncCountdown (from, delay = 1000) {
for (let i = from; i >= 0; i--) {
yield i
}
}
loige 63

const countdown = createAsyncCountdown(3)
for await (const value of countdown) {
console.log(value)
}
loige 64

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.
65

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...
66

A REAL USE CASE & A CHALLENGE!
loige
Async Iterables are commonly used to abstract over paginated resources. Can you implement a
paginator for the Rick & Morty API?
function createCharactersPaginator () {
// return an async iterable object 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)
}
67

IN CONCLUSION 💥
loige
JavaScript has built-in sync/async iteratable/iterators
These are great tools when you need to deal with large (or even endless) data sets
They give you great control over how to consume data incrementally
Iterator helpers are still lacking and not as mature as in other languages
... although, you should know enough to be able to create your own helpers now 😜
... and you can leverage generator functions to make your life easier!
... or you can wait for & to land
Iterator Helpers Async Iterator Helpers
... or you can use to polyﬁll these features today!
core-js
68

BONUS: A FREE WORKSHOP FOR YOU! 🎁
loige
loige.link/lets-iter-repo
git clone https://github.com/lmammino/iteration-protocols-workshop.git
cd iteration-protocols-workshop
npm i
69

Front cover Photo by on
Back cover photo by on
Sam Barber Unsplash
photonblast Unsplash
fourtheorem.com
THANKS! 🙌
loige
nodejsdp.link
70

Did you know JavaScript has iterators? DublinJS

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