What's wrong with this use of async await? - node.js

Question

I am trying to download tracks via the soundcloud API, and then launch a callback once an indeterminant amount of tracks is downloaded. When I run the below code, I see "All done" being console logged before anything else, even though I intend for it to be the last thing... What am I doing wrong?
// Deps
import fs from 'fs'
import SC from 'node-soundcloud'
import request from 'request'
// Write mp3 function
function writeMP3(track) {
return new Promise((resolve, reject) => {
console.log('Starting download: ', track.title)
request.get(track.download_url)
.on('error', err => {
// reject('Download error: ', err)
})
.on('finish', () => {
() => resolve('Download complete')
})
.pipe(fs.createWriteStream(`./data/temp/${track.title}_${track.user.username}.mp3`))
})
}
async function asyncTrackFetch(track) {
return await writeMP3(track)
}
// Array of promises to callback upon
const trackActions = []
SC.init({
id: 'MY_ID',
secret: 'MY_SECRET'
})
SC.get('/tracks', (err, tracks) => {
if (err) {
throw new Error(err)
} else {
console.log('Tracks fetched: ', tracks.length)
tracks.map(track => {
if (track.downloadable) {
console.log('downloadable')
trackActions.push(asyncTrackFetch(track))
}
})
}
})
// Perform requests async
Promise.all(trackActions).then(() => {
console.log('All done')
console.log(fs.readdirSync('./data/temp'))
})

Answer 1

Promise.all(trackActions) waits on whatever promises are in trackActions, but trackActions is empty at the time you make the call. You're only adding promises to the array after your SC.get callback gets called.
Try putting your Promise.all... block inside the SC.get callback like this:
SC.get('/tracks', (err, tracks) => {
if (err) {
throw new Error(err)
} else {
console.log('Tracks fetched: ', tracks.length)
tracks.map(track => {
if (track.downloadable) {
console.log('downloadable')
trackActions.push(asyncTrackFetch(track))
}
})
Promise.all(trackActions).then(() => {
console.log('All done')
console.log(fs.readdirSync('./data/temp'))
})
}
})
It's worth mentioning as well that your line throw new Error(err) will crash the program since there's nowhere for that error to be caught.
As Antonio Val mentioned, there are better ways to do this. If you promisify the node-soundcloud library then the last part of your code could look like this:
SC.get('/tracks').then(tracks => {
// No need for trackedActions array.
return Promise.all(tracks.filter(track => track.downloadable)
.map(track => asyncTrackFetch(track)))
}).then(fetchedTracks => {
console.log('All done fetching tracks', fetchedTracks)
}).catch(err => {
// Handle error.
})
Or inside an async function,
try {
const tracks = await SC.get('/tracks')
const fetchPromises = tracks
.filter(track => track.downloadable)
.map(track => asyncTrackFetch(track))
const fetchedTracks = await Promise.all(fetchPromises)
console('All done fetching tracks.', fetchedTracks)
} catch (err) {
// Handle error
}

Answer 2

I think the easiest way would be to move Promise.all after tracks.map loop finished.
A more elegant solution would be to promisify SC.get as well and use async await along all your code.
UPDATE:
Couldn't test it so not sure if it works, but it would be something like this:
import fs from 'fs'
import SC from 'node-soundcloud'
import request from 'request'
function writeMP3(track) {
return new Promise((resolve, reject) => {
console.log('Starting download: ', track.title)
request.get(track.download_url)
.on('error', err => {
// reject('Download error: ', err)
})
.on('finish', () => {
() => resolve('Download complete')
})
.pipe(fs.createWriteStream(`./data/temp/${track.title}_${track.user.username}.mp3`))
})
}
function getTracks() {
return new Promise((resolve, reject) => {
SC.get('/tracks', (err, tracks) => {
if (err) {
return reject(err)
}
console.log('Tracks fetched: ', tracks.length)
resolve(tracks)
})
})
}
SC.init({
id: 'MY_ID',
secret: 'MY_SECRET'
})
With async await:
async function start() {
const tracks = await getTracks();
for (let track of tracks) {
await writeMP3(track)
}
}
start()
.then(() => {
console.log('All done')
console.log(fs.readdirSync('./data/temp'))
})
.catch((err) => {
// insert error handler here
})
If you just want to use Promises:
getTracks
.then((tracks) => {
const promiseArray = tracks.map((track) => {
return writeMP3(track)
})
return Promise.all(promiseArray)
})
.then(() => {
console.log('All done')
console.log(fs.readdirSync('./data/temp'))
})
.catch((err) => {
// insert error handler here
})