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Finding a lost song with Node.js & async iterators Luciano Mammino ( ) @loige 2021-10-18 loige.link/nodeconf-iter 1

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Get these slides! loige loige.link/nodeconf-iter 2

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Photo by on Darius Bashar Unsplash A random song you haven't listened to in years pops into your head... πŸ‘‚πŸ› 3

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It doesn't matter what you do all day... It keeps coming back to you! πŸ› Photo by on Attentie Attentie Unsplash 4

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And now you want to listen to it! Photo by on Volodymyr Hryshchenko Unsplash 5

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But, what if you can't remember the title or the author?! Photo by on Tachina Lee Unsplash 6

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THERE MUST BE A WAY TO REMEMBER! Photo by on Marius Niveri Unsplash 7

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Today, I'll tell you how I solved this problem using - Last.fm API - Node.js - Async Iterators Photo by on Quinton Coetzee Unsplash 8

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Let me introduce myself first... 9

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Let me introduce myself first... πŸ‘‹ I'm Luciano ( πŸ•πŸ) 9

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Let me introduce myself first... πŸ‘‹ I'm Luciano ( πŸ•πŸ) Senior Architect @ fourTheorem (Dublin ) 9

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Let me introduce myself first... πŸ‘‹ I'm Luciano ( πŸ•πŸ) Senior Architect @ fourTheorem (Dublin ) nodejsdp.link πŸ“” Co-Author of Node.js Design Patterns πŸ‘‰ 9

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Let me introduce myself first... πŸ‘‹ I'm Luciano ( πŸ•πŸ) Senior Architect @ fourTheorem (Dublin ) nodejsdp.link πŸ“” Co-Author of Node.js Design Patterns πŸ‘‰ Let's connect! (blog) (twitter) (twitch) (github) loige.co @loige loige lmammino 9

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We are business focused technologists that deliver. | | Accelerated Serverless AI as a Service Platform Modernisation We are hiring: do you want to ? work with us loige 10

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So, there was this song in my mind... πŸ› loige 11

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I could only remember some random parts and the word "dark" (probably in the title) loige 12

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Luciano - scrobbling since 12 Feb 2007 loige 15

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Luciano - scrobbling since 12 Feb 2007 loige ~250k scrobbles... that song must be there! 15

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loige There's an API! https://www.last.fm/api 16

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loige Let's give it a shot curl "http://ws.audioscrobbler.com/2.0/? method=user.getrecenttracks&user=loige&api_key =${API_KEY}&format=json" | jq . 18

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It works! πŸ₯³ Now let's do this with JavaScript loige 20

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import { request } from 'undici' const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json' }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() console.log(data) loige 21

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import { request } from 'undici' const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json' }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() console.log(data) loige 21

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import { request } from 'undici' const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json' }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() console.log(data) loige 21

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import { request } from 'undici' const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json' }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() console.log(data) loige 21

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import { request } from 'undici' const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json' }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() console.log(data) loige 21

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loige We are getting a "paginated" response with 50 tracks per page 22

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loige We are getting a "paginated" response with 50 tracks per page but there are 51 here! πŸ€” 22

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loige We are getting a "paginated" response with 50 tracks per page but there are 51 here! πŸ€” (let's ignore this for now...) 22

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loige We are getting a "paginated" response with 50 tracks per page but there are 51 here! πŸ€” How do we fetch the next pages? (let's ignore this for now...) 22

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let page = 1 while (true) { const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', page }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() console.log(data) if (page === Number(data.recenttracks['@attr'].totalPages)) { break // it's the last page! } loige 24

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let page = 1 while (true) { const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', page }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() console.log(data) if (page === Number(data.recenttracks['@attr'].totalPages)) { break // it's the last page! } loige 24

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let page = 1 while (true) { const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', page }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() console.log(data) if (page === Number(data.recenttracks['@attr'].totalPages)) { break // it's the last page! } loige 24

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let page = 1 while (true) { const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', page }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() console.log(data) if (page === Number(data.recenttracks['@attr'].totalPages)) { break // it's the last page! } loige 24

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let page = 1 while (true) { const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', page }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() console.log(data) if (page === Number(data.recenttracks['@attr'].totalPages)) { break // it's the last page! } loige 24

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let page = 1 while (true) { const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', page }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() console.log(data) if (page === Number(data.recenttracks['@attr'].totalPages)) { break // it's the last page! } loige 24

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Seems good! πŸ‘Œ Let's look at the tracks... loige 26

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// ... for (const track of data.recenttracks.track) { console.log( track.date?.['#text'], `${track.artist['#text']} - ${track.name}` ) } console.log('--- end page ---') // ... loige 27

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loige * Note that page size here is 10 tracks per page 28

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loige * Note that page size here is 10 tracks per page Every page has a song with undefined time... This is the song I am currently listening to! It appears at the top of every page. 28

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loige * Note that page size here is 10 tracks per page Sometimes there are duplicated tracks between pages... 😨 28

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The "sliding windows" problem 😩 loige 29

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loige ... tracks (newest to oldest) 30

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loige ... tracks (newest to oldest) 30 Page1 Page2

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loige ... tracks (newest to oldest) 30 Page1 Page2 ...

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loige ... tracks (newest to oldest) 30 Page1 Page2 ... new track

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loige ... tracks (newest to oldest) 30 Page1 Page2 ... Page1 Page2 new track

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loige ... tracks (newest to oldest) 30 Page1 Page2 ... Page1 Page2 new track moved from page 1 to page 2

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Time based windows 😎 loige 32

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loige ...* tracks (newest to oldest) 33 * we are done when we get an empty page (or num pages is 1) to ... from

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loige ...* tracks (newest to oldest) 33 Page1 * we are done when we get an empty page (or num pages is 1) to ... from

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loige ...* tracks (newest to oldest) 33 Page1 t1 * we are done when we get an empty page (or num pages is 1) to ... from

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loige ...* tracks (newest to oldest) 33 Page1 before t1 (page 1 "to" t1) t1 * we are done when we get an empty page (or num pages is 1) to ... from

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loige ...* tracks (newest to oldest) 33 Page1 before t1 (page 1 "to" t1) t1 t2 * we are done when we get an empty page (or num pages is 1) to ... from

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loige ...* tracks (newest to oldest) 33 Page1 before t1 (page 1 "to" t1) t1 t2 before t2 (page 1 "to" t2) * we are done when we get an empty page (or num pages is 1) to ... from

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let to = '' while (true) { const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', limit: '10', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() const tracks = data.recenttracks.track console.log( `--- ↓ page to ${to}`, `remaining pages: ${data.recenttracks['@attr'].totalPages} ---` ) for (const track of tracks) { console.log(track.date?.uts, `${track.artist['#text']} - ${track.name}`) } if (data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } loige 34

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let to = '' while (true) { const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', limit: '10', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() const tracks = data.recenttracks.track console.log( `--- ↓ page to ${to}`, `remaining pages: ${data.recenttracks['@attr'].totalPages} ---` ) for (const track of tracks) { console.log(track.date?.uts, `${track.artist['#text']} - ${track.name}`) } if (data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } loige 34

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let to = '' while (true) { const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', limit: '10', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() const tracks = data.recenttracks.track console.log( `--- ↓ page to ${to}`, `remaining pages: ${data.recenttracks['@attr'].totalPages} ---` ) for (const track of tracks) { console.log(track.date?.uts, `${track.artist['#text']} - ${track.name}`) } if (data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } loige 34

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let to = '' while (true) { const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', limit: '10', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() const tracks = data.recenttracks.track console.log( `--- ↓ page to ${to}`, `remaining pages: ${data.recenttracks['@attr'].totalPages} ---` ) for (const track of tracks) { console.log(track.date?.uts, `${track.artist['#text']} - ${track.name}`) } if (data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } loige 34

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let to = '' while (true) { const query = new URLSearchParams({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', limit: '10', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() const tracks = data.recenttracks.track console.log( `--- ↓ page to ${to}`, `remaining pages: ${data.recenttracks['@attr'].totalPages} ---` ) for (const track of tracks) { console.log(track.date?.uts, `${track.artist['#text']} - ${track.name}`) } if (data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } loige 34

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loige The track of the last timestamp becomes the boundary for the next page 35

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We have a working solution! πŸŽ‰ Can we generalise it? loige 36

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We know how to iterate over every page/track. How do we expose this information? loige const reader = LastFmRecentTracks({ apikey: process.env.API_KEY, user: 'loige' }) 37

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// callbacks reader.readPages( (page) => { /* ... */ }, // on page (err) => { /* ... */} // on completion (or error) ) loige 38

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// event emitter reader.read() reader.on('page', (page) => { /* ... */ }) reader.on('completed', (err) => { /* ... */ }) loige 39

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// streams ❀ reader.pipe(/* transform or writable stream here */) reader.on('end', () => { /* ... */ }) reader.on('error', () => { /* ... */ }) loige 40

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// streams pipeline ❀ ❀ pipeline( reader, yourProcessingStream, (err) => { // handle completion or err } ) loige 41

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// ASYNC ITERATORS! 😡 for await (const page of reader) { /* ... */ } // ... do more stuff when all the // data is consumed loige 42

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// ASYNC ITERATORS WITH ERROR HANDLING! 🀯 try { for await (const page of reader) { /* ... */ } } catch (err) { // handle errors } // ... do more stuff when all the // data is consumed loige 43

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How can we build an async iterator? 🧐 loige 44

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Meet the iteration protocols! loige loige.co/javascript-iterator-patterns 45

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Iteration concepts loige Iterator An object that acts as a cursor to iterate over blocks of data sequentially Iterable An object that contains data that can be iterated over sequentially ... ... 46

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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 47

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function createCountdown (from) { let nextVal = from return { next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } } } loige 48

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function createCountdown (from) { let nextVal = from return { next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } } } loige 48

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function createCountdown (from) { let nextVal = from return { next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } } } loige 48

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function createCountdown (from) { let nextVal = from return { next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } } } loige 48

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function createCountdown (from) { let nextVal = from return { next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } } } loige 48

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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 49

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Generator functions "produce" iterators! loige 50

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function * createCountdown (from) { for (let i = from; i >= 0; i--) { yield i } } loige 51

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function * createCountdown (from) { for (let i = from; i >= 0; i--) { yield i } } loige 51

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function * createCountdown (from) { for (let i = from; i >= 0; i--) { yield i } } loige 51

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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 52

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The iterable protocol An object is iterable if it implements the Symbol.iterator method, a zero-argument function that returns an iterator. loige 53

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function createCountdown (from) { let nextVal = from return { [Symbol.iterator]: () => ({ next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } }) } } loige 54

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function createCountdown (from) { let nextVal = from return { [Symbol.iterator]: () => ({ next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } }) } } loige 54

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function createCountdown (from) { let nextVal = from return { [Symbol.iterator]: () => ({ next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } }) } } loige 54

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function createCountdown (from) { return { [Symbol.iterator]: function * () { for (let i = from; i >= 0; i--) { yield i } } } } loige 55

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const countdown = createCountdown(3) for (const value of countdown) { console.log(value) } // 3 // 2 // 1 // 0 loige 56

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OK. So far this is all synchronous iteration. What about async? πŸ™„ loige 57

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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 58

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import { setTimeout } from '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 59

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import { setTimeout } from '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 59

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import { setTimeout } from '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 59

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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 60

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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 60

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import { setTimeout } from '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 62

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import { setTimeout } from '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 62

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import { setTimeout } from '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 62

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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 63

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import { setTimeout } from '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 64

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import { setTimeout } from '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 64

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HOT TIP πŸ”₯ With async generators we can create objects that are both async iterators and async iterables! loige (We don't need to specify Symbol.asyncIterator explicitly!) 65

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import { setTimeout } from '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 66

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const countdown = createAsyncCountdown(3) for await (const value of countdown) { console.log(value) } loige 67

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const countdown = createAsyncCountdown(3) for await (const value of countdown) { console.log(value) } loige 67

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Now we know how to make our LastFmRecentTracks an Async Iterable 🀩 loige 68

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import { request } from 'undici' async function* createLastFmRecentTracks (apiKey, user) { let to = '' while (true) { const query = new URLSearchParams({ method: 'user.getrecenttracks', user, api_key: apiKey, format: 'json', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() const tracks = data.recenttracks.track yield tracks if (data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } } loige 69

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import { request } from 'undici' async function* createLastFmRecentTracks (apiKey, user) { let to = '' while (true) { const query = new URLSearchParams({ method: 'user.getrecenttracks', user, api_key: apiKey, format: 'json', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() const tracks = data.recenttracks.track yield tracks if (data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } } loige 69

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import { request } from 'undici' async function* createLastFmRecentTracks (apiKey, user) { let to = '' while (true) { const query = new URLSearchParams({ method: 'user.getrecenttracks', user, api_key: apiKey, format: 'json', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const { body } = await request(url) const data = await body.json() const tracks = data.recenttracks.track yield tracks if (data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } } loige 69

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const recentTracks = createLastFmRecentTracks( process.env.API_KEY, 'loige' ) for await (const page of recentTracks) { console.log(page) } loige 70

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Let's search for all the songs that contain the word "dark" in their title! 🧐 loige 71

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const recentTracks = createLastFmRecentTracks( process.env.API_KEY, 'loige' ) for await (const page of recentTracks) { for (const track of page) { if (track.name.toLowerCase().includes('dark')) { console.log(`${track.artist['#text']} - ${track.name}`) } } } loige 72

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loige OMG! This is the song! 😱 ...from 8 years ago! 73

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For a more serious package that allows you to fetch data from Last.fm: loige npm install scrobbles 74

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Cover Picture by on ❀ Thanks to Jacek Spera, , , , , for reviews and suggestions. FPVmat A Unsplash @eoins @pelger @gbinside @ManuEomm @simonplend - loige.link/nodeconf-iter loige.link/async-it-code for await (const _ of createAsyncCountdown(1_000_000)) { console.log("THANK YOU! 😍 ") } loige nodejsdp.link 75