Finding a lost song
with Node.js & async iterators

Luciano Mammino (@loige)

Global Summit for Node.js

2022-05-18

loige.link/async-iter-song

Get these slides!

loige

loige.link/async-iter-song

Photo by Darius Bashar on Unsplash

 A random song you haven't listened to
in years pops into your head...

👂🐛

It doesn't matter what you do all day...
It keeps coming back to you! 🐛

Photo by Attentie Attentie on Unsplash

And now you want to listen to it!

Photo by Volodymyr Hryshchenko on Unsplash

But, what if you can't remember

the title or the author?!

Photo by Tachina Lee on Unsplash

THERE MUST BE A WAY TO REMEMBER!

Photo by Marius Niveri on Unsplash

Today, I'll tell you how I solved this problem using

- Last.fm API

- Node.js

- Async Iterators

Photo by Quinton Coetzee on Unsplash

Let me introduce myself first...

👋 I'm Luciano (🇮🇹🍕🍝🤌)

👨‍💻 Senior Architect @ fourTheorem (Dublin 🇮🇪)

nodejsdp.link

📔 Co-Author of Node.js Design Patterns  👉

Let's connect!

  loige.co (blog)

  @loige (twitter)

  loige (twitch)

  lmammino (github)

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

So, there was this song in my mind... 🐛

loige

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

loige

Luciano - scrobbling since 12 Feb 2007

loige

~250k scrobbles... that song must be there!

loige

There's an API!

https://www.last.fm/api

loige

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 .

loige

It works! 🥳

Now let's do this with JavaScript

loige

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

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...)

loige

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!
  }

  page++
}

loige

loige

Seems good! 👌

Let's look at the tracks...

loige

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

loige

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.

Sometimes there are duplicated tracks between pages... 😨

The "sliding windows" problem 😩

loige

loige

...

tracks (newest to oldest)

image/svg+xml
image/svg+xml

Page1

Page2

...

image/svg+xml
image/svg+xml

Page1

Page2

new track

moved from page 1 to page 2

loige

Time based windows 😎

loige

loige

...*

tracks (newest to oldest)

image/svg+xml
image/svg+xml

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)

image/svg+xml

to             ...          from

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

loige

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

We have a working solution! 🎉
Can we generalise it?

loige

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'
})
// callbacks

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

loige

// event emitter

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

loige

// streams ❤️

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

loige

// streams pipeline ❤️❤️

pipeline(
  reader,
  yourProcessingStream,
  (err) => {
    // handle completion or err
  }
)

loige

// ASYNC ITERATORS! 😵


for await (const page of reader) {
  /* ... */
}

// ... do more stuff when all the 
// data is consumed

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

How can we build an async iterator? 🧐

loige

Meet the iteration protocols!

loige

loige.co/javascript-iterator-patterns

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

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

function createCountdown (from) {
  let nextVal = from
  return {
    next () {
      if (nextVal < 0) {
        return { done: true }
      }

      return { 
        done: false,
        value: nextVal--
      }
    }
  }
}

loige

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

Generator functions "produce" iterators!

loige

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

loige

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

The iterable protocol

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

loige

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

        return { done: false, value: nextVal-- }
      }
    })
  }
}

loige

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

loige

const countdown = createCountdown(3)

for (const value of countdown) {
  console.log(value)
}

// 3
// 2
// 1
// 0

loige

OK. So far this is all synchronous iteration.
What about async? 🙄

loige

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

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

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

loige

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

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

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

HOT TIP 🔥

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!)
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

const countdown = createAsyncCountdown(3)

for await (const value of countdown) {
  console.log(value)
}

loige

Now we know how to make our LastFmRecentTracks an Async Iterable 🤩

loige

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

const recentTracks = createLastFmRecentTracks(
  process.env.API_KEY,
  'loige'
)

for await (const page of recentTracks) {
  console.log(page)
}

loige

loige

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

loige

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

loige

OMG! This is the song! 😱
...from 9 years ago!

🔥 HOT TIP

You can make your Async Iterators "smart" by adding extra behaviours (e.g. error handling and automatic retries)

loige

loige.link/scrobbles

Photo by Lee Campbell on Unsplash
❤️ Thanks to @goldbergyoni, Jacek Spera, @eoins, @pelger, @gbinside, @ManuEomm, @simonplend  for reviews and suggestions.

loige.link/async-iter-song​  -  loige.link/async-it-code - loige.link/scrobbles

for await (const _ of createAsyncCountdown(1_000_000)) {
  console.log("THANK YOU! 😍")
}

loige

nodejsdp.link