Finding a lost song with Node.js and async iterators

Did you ever get that feeling when a random song pops into your brain and you can’t get rid of it? Well, that happened to me recently and I couldn’t even remember the title of the damn song! In this talk, I want to share with you the story of how I was able to recover the details of the song by navigating some music-related APIs using JavaScript, Node.js and the magic of async iterators!

1. Finding a lost song with Node.js & async iterators Luciano Mammino ( ) @loige Dublin, Node.js Meetup 2021-04-29 loige.link/async-it 1
2. loige.link/async-it Get these slides! loige 2
3. Photo by on Darius Bashar Unsplash A random song you haven't listened to in years pops into your head... 3
4. It doesn't matter what you do all day... It keeps coming back to you! Photo by on Attentie Attentie Unsplash 4
5. And now you want to listen to it! Photo by on Volodymyr Hryshchenko Unsplash 5
6. But, what if you can't remember the title or the author?! Photo by on Tachina Lee Unsplash 6
7. THERE MUST BE A WAY TO REMEMBER! Photo by on Marius Niveri Unsplash 7
8. 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
9. Let me introduce myself first... 9
10. Let me introduce myself first... I'm Luciano ( 🍕🍝) 👋 9
11. Let me introduce myself first... I'm Luciano ( 🍕🍝) 👋 Senior Architect @ fourTheorem (Dublin ) 9
12. Let me introduce myself first... I'm Luciano ( 🍕🍝) 👋 Senior Architect @ fourTheorem (Dublin ) nodejsdp.link Co-Author of Node.js Design Patterns 👉 9
13. Let me introduce myself first... I'm Luciano ( 🍕🍝) 👋 Senior Architect @ fourTheorem (Dublin ) nodejsdp.link Co-Author of Node.js Design Patterns 👉 20% eBook discount on Packt 20NODEDUBLIN 9
14. Let me introduce myself first... I'm Luciano ( 🍕🍝) 👋 Senior Architect @ fourTheorem (Dublin ) nodejsdp.link Co-Author of Node.js Design Patterns 👉 Connect with me: (blog) (twitter) (github) loige.co @loige lmammino 20% eBook discount on Packt 20NODEDUBLIN 9
15. We are business focused technologists that deliver. | | Accelerated Serverless AI as a Service Platform Modernisation Do you want to ? work with us loige 10
16. There was this song in my mind... loige 11
17. I could only remember some random parts and the word "dark" (probably in the title) loige 12
20. loige 15
21. Luciano - scrobbling since 12 Feb 2007 loige 15
22. Luciano - scrobbling since 12 Feb 2007 loige ~250k scrobbles... that song must be there! 15
23. loige 16
24. loige ~5k pages of history & no search functionality! 😓 16
25. loige But there's an API! https://www.last.fm/api 17
26. loige 18
27. loige Let's give it a shot curl "http://ws.audioscrobbler.com/2.0/? method=user.getrecenttracks&user=loige&api_key =${API_KEY}&format=json" | jq . 19
28. loige 20
29. It works! 🥳 Let's convert this to JavaScript loige 21
30. import querystring from 'querystring' import axios from 'axios' const query = querystring.stringify({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json' }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const response = await axios.get(url) console.log(response.data) loige 22
31. loige 23
32. loige We are getting a "paginated" response with 50 tracks per page 23
33. loige We are getting a "paginated" response with 50 tracks per page but there are 51 here! 🤔 23
34. loige We are getting a "paginated" response with 50 tracks per page but there are 51 here! 🤔 (let's ignore this for now...) 23
35. loige We are getting a "paginated" response with 50 tracks per page but there are 51 here! 🤔 How do we fetch the next pages? (let's ignore this for now...) 23
36. loige 24
37. let page = 1 while (true) { const query = querystring.stringify({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', page }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const response = await axios.get(url) console.log(response.data) if (page === Number(response.data.recenttracks['@attr'].totalPages)) { break // it's the last page! } loige 25
38. let page = 1 while (true) { const query = querystring.stringify({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', page }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const response = await axios.get(url) console.log(response.data) if (page === Number(response.data.recenttracks['@attr'].totalPages)) { break // it's the last page! } loige 25
39. let page = 1 while (true) { const query = querystring.stringify({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', page }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const response = await axios.get(url) console.log(response.data) if (page === Number(response.data.recenttracks['@attr'].totalPages)) { break // it's the last page! } loige 25
40. let page = 1 while (true) { const query = querystring.stringify({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', page }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const response = await axios.get(url) console.log(response.data) if (page === Number(response.data.recenttracks['@attr'].totalPages)) { break // it's the last page! } loige 25
41. let page = 1 while (true) { const query = querystring.stringify({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', page }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const response = await axios.get(url) console.log(response.data) if (page === Number(response.data.recenttracks['@attr'].totalPages)) { break // it's the last page! } loige 25
42. let page = 1 while (true) { const query = querystring.stringify({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', page }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const response = await axios.get(url) console.log(response.data) if (page === Number(response.data.recenttracks['@attr'].totalPages)) { break // it's the last page! } loige 25
43. loige 26
44. loige 26
45. Seems good! Let's look at the tracks... loige 27
46. // ... for (const track of response.data.recenttracks.track) { console.log( track.date?.['#text'], `${track.artist['#text']} - ${track.name}` ) } console.log('--- end page ---') // ... loige 28
47. loige 29
48. loige * Note that page size here is 10 tracks per page 29
49. loige * Note that page size here is 10 tracks per page Every page has a song with undefined time... This is the song I am currently listening to! It appears at the top of every page. 29
50. loige * Note that page size here is 10 tracks per page Sometimes there are duplicated tracks between pages... 😨 29
51. The "sliding windows" problem 😩 loige 30
52. loige ... tracks (newest to oldest) 31
53. loige ... tracks (newest to oldest) 31 Page1 Page2
54. loige ... tracks (newest to oldest) 31 Page1 Page2 ...
55. loige ... tracks (newest to oldest) 31 Page1 Page2 ... new track
56. loige ... tracks (newest to oldest) 31 Page1 Page2 ... Page1 Page2 new track
57. loige ... tracks (newest to oldest) 31 Page1 Page2 ... Page1 Page2 new track moved from page 1 to page 2
58. loige 32
59. Time based windows 😎 loige 33
60. loige ...* tracks (newest to oldest) 34 * we are done when we get an empty page (or num pages is 1)
61. loige ...* tracks (newest to oldest) 34 Page1 * we are done when we get an empty page (or num pages is 1)
62. loige ...* tracks (newest to oldest) 34 Page1 t1 * we are done when we get an empty page (or num pages is 1)
63. loige ...* tracks (newest to oldest) 34 Page1 before t1 (page 1 "to" t1) t1 * we are done when we get an empty page (or num pages is 1)
64. loige ...* tracks (newest to oldest) 34 Page1 before t1 (page 1 "to" t1) t1 t2 * we are done when we get an empty page (or num pages is 1)
65. loige ...* tracks (newest to oldest) 34 Page1 before t1 (page 1 "to" t1) t1 t2 before t2 (page 1 "to" t2) * we are done when we get an empty page (or num pages is 1)
66. let to while (true) { const query = querystring.stringify({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', limit: '10', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const response = await axios.get(url) const tracks = response.data.recenttracks.track console.log( `--- ↓ page to ${to}`, `remaining pages: ${response.data.recenttracks['@attr'].totalPages} ---` ) for (const track of tracks) { console.log(track.date?.uts, `${track.artist['#text']} - ${track.name}`) } if (response.data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } loige 35
67. let to while (true) { const query = querystring.stringify({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', limit: '10', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const response = await axios.get(url) const tracks = response.data.recenttracks.track console.log( `--- ↓ page to ${to}`, `remaining pages: ${response.data.recenttracks['@attr'].totalPages} ---` ) for (const track of tracks) { console.log(track.date?.uts, `${track.artist['#text']} - ${track.name}`) } if (response.data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } loige 35
68. let to while (true) { const query = querystring.stringify({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', limit: '10', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const response = await axios.get(url) const tracks = response.data.recenttracks.track console.log( `--- ↓ page to ${to}`, `remaining pages: ${response.data.recenttracks['@attr'].totalPages} ---` ) for (const track of tracks) { console.log(track.date?.uts, `${track.artist['#text']} - ${track.name}`) } if (response.data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } loige 35
69. let to while (true) { const query = querystring.stringify({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', limit: '10', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const response = await axios.get(url) const tracks = response.data.recenttracks.track console.log( `--- ↓ page to ${to}`, `remaining pages: ${response.data.recenttracks['@attr'].totalPages} ---` ) for (const track of tracks) { console.log(track.date?.uts, `${track.artist['#text']} - ${track.name}`) } if (response.data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } loige 35
70. let to while (true) { const query = querystring.stringify({ method: 'user.getrecenttracks', user: 'loige', api_key: process.env.API_KEY, format: 'json', limit: '10', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const response = await axios.get(url) const tracks = response.data.recenttracks.track console.log( `--- ↓ page to ${to}`, `remaining pages: ${response.data.recenttracks['@attr'].totalPages} ---` ) for (const track of tracks) { console.log(track.date?.uts, `${track.artist['#text']} - ${track.name}`) } if (response.data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } loige 35
71. loige 36
72. loige The track of the last timestamp becomes the boundary for the next page 36
73. We have a working solution! 🎉 Can we generalise it? loige 37
74. We know how to iterate over every page/track. How do we expose this information? loige 38
75. const reader = LastFmRecentTracks({ apikey: process.env.API_KEY, user: 'loige' }) // callbacks reader.readPages( (page) => { /* ... */ }, // on page (err) => { /* ... */} // on completion (or error) ) loige 39
76. const reader = LastFmRecentTracks({ apikey: process.env.API_KEY, user: 'loige' }) // event emitter reader.read() reader.on('page', (page) => { /* ... */ }) reader.on('completed', (err) => { /* ... */ }) loige 40
77. const reader = LastFmRecentTracks({ apikey: process.env.API_KEY, user: 'loige' }) // streams <3 reader.pipe(/* transform or writable stream here */) reader.on('end', () => { /* ... */ }) reader.on('error', () => { /* ... */ }) loige 41
78. import { pipeline } from 'stream' const reader = LastFmRecentTracks({ apikey: process.env.API_KEY, user: 'loige' }) // streams pipeline <3 <3 pipeline( reader, yourProcessingStream, (err) => { // handle completion or err } ) loige 42
79. const reader = LastFmRecentTracks({ apikey: process.env.API_KEY, user: 'loige' }) // ASYNC ITERATORS! for await (const page of reader) { /* ... */ } // ... do more stuff when all the data is consumed loige 43
80. const reader = LastFmRecentTracks({ apikey: process.env.API_KEY, user: 'loige' }) // 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 44
81. How can we build an async iterator? 🧐 loige 45
82. Meet the iteration protocols! loige loige.co/javascript-iterator-patterns 46
83. 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
84. function createCountdown (from) { let nextVal = from return { next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } } } loige 48
85. function createCountdown (from) { let nextVal = from return { next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } } } loige 48
86. function createCountdown (from) { let nextVal = from return { next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } } } loige 48
87. function createCountdown (from) { let nextVal = from return { next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } } } loige 48
88. function createCountdown (from) { let nextVal = from return { next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } } } loige 48
89. 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
90. Generator functions "produce" iterators! loige 50
91. function * createCountdown (from) { for (let i = from; i >= 0; i--) { yield i } } loige 51
92. function * createCountdown (from) { for (let i = from; i >= 0; i--) { yield i } } loige 51
93. function * createCountdown (from) { for (let i = from; i >= 0; i--) { yield i } } loige 51
94. 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
95. The iterable protocol An object is iterable if it implements the @@iterator* method, a zero-argument function that returns an iterator. loige *Symbol.iterator 53
96. function createCountdown (from) { let nextVal = from return { [Symbol.iterator]: () => ({ next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } }) } } loige 54
97. function createCountdown (from) { let nextVal = from return { [Symbol.iterator]: () => ({ next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } }) } } loige 54
98. function createCountdown (from) { let nextVal = from return { [Symbol.iterator]: () => ({ next () { if (nextVal < 0) { return { done: true } } return { done: false, value: nextVal-- } } }) } } loige 54
99. function createCountdown (from) { return { [Symbol.iterator]: function * () { for (let i = from; i >= 0; i--) { yield i } } } } loige 55
100. const countdown = createCountdown(3) for (const value of countdown) { console.log(value) } // 3 // 2 // 1 // 0 loige 56
101. OK. So far this is all synchronous iteration. What about async? 🙄 loige 57
102. 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
103. 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
104. 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
105. 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
106. 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
107. 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
108. loige 61
109. 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
110. 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
111. 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
112. The async iterable protocol An object is an async iterable if it implements the @@asyncIterator* method, a zero-argument function that returns an async iterator. loige *Symbol.asyncIterator 63
113. 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
114. 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
115. const countdown = createAsyncCountdown(3) for await (const value of countdown) { console.log(value) } loige 65
116. const countdown = createAsyncCountdown(3) for await (const value of countdown) { console.log(value) } loige 65
117. Now we know how to make our LastFmRecentTracks an Async Iterable 🤩 loige 66
118. function createLastFmRecentTracks (apiKey, user) { return { [Symbol.asyncIterator]: async function * () { let to while (true) { const query = querystring.stringify({ method: 'user.getrecenttracks', user, api_key: apiKey, format: 'json', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const response = await axios.get(url) const tracks = response.data.recenttracks.track yield tracks if (response.data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } } } } loige 67
119. function createLastFmRecentTracks (apiKey, user) { return { [Symbol.asyncIterator]: async function * () { let to while (true) { const query = querystring.stringify({ method: 'user.getrecenttracks', user, api_key: apiKey, format: 'json', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const response = await axios.get(url) const tracks = response.data.recenttracks.track yield tracks if (response.data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } } } } loige 67
120. function createLastFmRecentTracks (apiKey, user) { return { [Symbol.asyncIterator]: async function * () { let to while (true) { const query = querystring.stringify({ method: 'user.getrecenttracks', user, api_key: apiKey, format: 'json', to }) const url = `https://ws.audioscrobbler.com/2.0/?${query}` const response = await axios.get(url) const tracks = response.data.recenttracks.track yield tracks if (response.data.recenttracks['@attr'].totalPages <= 1) { break // it's the last page! } const lastTrackInPage = tracks[tracks.length - 1] to = lastTrackInPage.date.uts } } } } loige 67
121. const recentTracks = createLastFmRecentTracks( process.env.API_KEY, 'loige' ) for await (const page of recentTracks) { console.log(page) } loige 68
122. Let's search for all the songs that contain the word "dark" in their title! 🧐 loige 69
123. async function main () { 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 70
124. loige 71
125. loige OMG! This is the song! 😱 ...from 8 years ago! 71
126. For a more serious package that allows you to fetch data from Last.fm: loige npm install scrobbles 72
127. Cover picture by on Thanks to Jacek Spera, , , , for reviews and suggestions. Rod Long Unsplash @eoins @pelger @gbinside @ManuEomm - loige.link/async-it loige.link/async-it-code for await (const _ of createAsyncCountdown(1_000_000)) { console.log("THANK YOU! 😍 ") } loige nodejsdp.link 20% eBook discount on Packt 20NODEDUBLIN 73