From Node.js to Design Patterns - BuildPiper

$ ~ whoami
👋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 2

Always re-imagining
We are a pioneering technology consultancy focused
on AWS and serverless
| |
Accelerated Serverless AI as a Service Platform Modernisation
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✉Reach out to us at
😇We are always looking for talent:
hello@fourTheorem.com
fth.link/careers
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We host a weekly podcast about AWS
awsbites.com
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Fact: Async JavaScript is tricky!
callbacks
promises
Async/Await
async
generators
streams
event emitters
util.promisify()
Promise.all()
Promise.allSettled()
😱
loige 5

Agenda
Async WUT?!
Callbacks
Promises
Async / Await
async Patterns
Mixed style async
A performance trick!
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What does async even mean?
In JavaScript and in Node.js, input/output operations are non-
blocking.
Classic examples: reading the content of a file, making an HTTP
request, loading data from a database, etc.
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Blocking style vs JavaScript
Blocking style JavaScript
1. Assign a variable
2. Read data from a file
3. Print to stdout
3. Print to stdout
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3. Print to stdout
3. Print to stdout
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3. Print to stdout
3. Print to stdout
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3. Print to stdout
3. Print to stdout
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3. Print to stdout
3. Print to stdout
(done)
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3. Print to stdout
3. Print to stdout
(done)
(done)
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Non-blocking I/O is convenient:
you can do work while waiting for I/O!
But, what if we need to do something when
the I/O operation completes?
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Once upon a time there were...
Callbacks
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Anatomy of callback-based non-blocking code
doSomethingAsync(arg1, arg2, cb)
This is a callback
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doSomethingAsync(arg1, arg2, (err, data) => {
// ... do something with data
})
You are defining what happens when the I/O operations
completes (or fails) with a function.
doSomethingAsync will call that function for you!
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doSomethingAsync(arg1, arg2, (err, data) => {
if (err) {
// ... handle error
return
}
})
Always handle errors first!
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An example
Fetch the latest booking for a given user
If it exists print it
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getLatestBooking(userId, (err, booking) => {
if (err) {
console.error(err)
return
}
if (booking) {
console.log(`Found booking for user ${userId}`, booking)
} else {
console.log(`No booking found for user ${userId}`)
}
})
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An example
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A more realistic example
If it exists, cancel it
If it was already paid for, refund the user
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if (err) {
console.error(err)
return
}
if (booking) {
cancelBooking(booking.id, (err) => {
if (err) {
console.error(err)
return
}
if (booking.paid) {
console.log('Booking was paid, refunding the user')
refundUser(userId, booking.paidAmount, (err) => {
if (err) {
console.error(err)
return
}
console.log('User refunded')
})
}
})
} else {
}
})
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if (err) {
console.error(err)
return
}
if (booking.paid) {
if (err) {
console.error(err)
return
}
})
}
})
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if (err) {
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console.error(err)
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return
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}
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if (booking) {
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} else {
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}
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})
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if (booking.paid) {
if (err) {
console.error(err)
return
}
})
}
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if (err) {
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console.error(err)
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return
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}
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if (booking) {
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if (err) {
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console.error(err)
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return
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}
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})
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} else {
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}
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})
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if (err) {
console.error(err)
return
}
})
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if (err) {
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console.error(err)
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return
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}
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if (booking) {
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if (err) {
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console.error(err)
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return
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}
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if (booking.paid) {
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}
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})
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} else {
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}
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})
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if (err) {
console.error(err)
return
}
if (booking) {
if (err) {
console.error(err)
return
}
if (booking.paid) {
if (err) {
console.error(err)
return
}
})
}
})
} else {
}
})
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if (err) {
console.error(err)
return
}
if (booking) {
if (err) {
console.error(err)
return
}
if (booking.paid) {
if (err) {
console.error(err)
return
}
})
}
})
} else {
}
})
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30 loige
THE PIRAMID OF
DOOM
(or callback hell 🔥)
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Some times, just
refactoring the code
can help...
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function cancelAndRefundBooking(booking, cb) {
if (err) { return cb(err) }
if (!booking.paid) {
return cb(null, {refundedAmount: 0})
}
refundUser(booking.userId, booking.paidAmount, (err) => {
if (err) { return cb(err) }
return cb(null, {refundedAmount: booking.paidAmount})
})
})
}
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if (err) {
console.error(err)
return
}
if (booking) {
cancelAndRefundBooking(booking, (err, result) => {
if (err) {
console.error(err)
return
}
console.log(`Booking cancelled (${result.refundedAmount} refunded)`)
})
}
})
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😟
Is this the best we can do?
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Let's talk about
Promise
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With callbacks we are not in
charge!
We need to trust that the async function will call our
callbacks when the async work is completed!
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Promise help us to be more in control!
const promiseObj = doSomethingAsync(arg1, arg2)
An object that represents the
status of the async operation
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A promise object is a tiny state machine with 2 possible states
pending (still performing the async operation)
settled (completed)
✅fullfilled (witha value)
🔥rejected (with an error)
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promiseObj.then((data) => {
})
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promiseObj.then((data) => {
})
promiseObj.catch((err) => {
// ... handle errors
}
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Promises can be chained ⛓
This solves the pyramid of doom problem!
doSomethingAsync(arg1, arg2)
.then((result) => doSomethingElseAsync(result))
.then((result) => doEvenMoreAsync(result)
.then((result) => keepDoingStuffAsync(result))
.catch((err) => { /* ... */ })
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loige

Promises can be chained ⛓
This solves the pyramid of doom problem!
doSomethingAsync(arg1, arg2)
.then((result) => doSomethingElseAsync(result))
// ...
.catch((err) => { /* ... */ })
.finally(() => { /* ... */ })
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How to create a promise
new Promise ((resolve, reject) => {
// ...
})
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new Promise ((resolve, reject) => {
// ... do something async
// reject(someError)
// resolve(someValue)
})
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Promise.resolve('SomeValue')
Promise.reject(new Error('SomeError'))
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How to create a promise (example)
function queryDB(client, query) {
return new Promise((resolve, reject) => {
client.executeQuery(query, (err, data) => {
if (err) {
return reject(err)
}
resolve(data)
})
})
}
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How to create a promise (example)
queryDB(dbClient, 'SELECT * FROM bookings')
.then((data) => {
})
.catch((err) => {
console.error('Failed to run query', err)
})
.finally(() => {
dbClient.disconnect()
})
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.then((data) => {
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})
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.catch((err) => {
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})
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.finally(() => {
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})
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.then((data) => {
})
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.catch((err) => {
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})
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.finally(() => {
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})
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.catch((err) => {
})
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.then((data) => {
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})
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.finally(() => {
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})
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.finally(() => {
})
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.then((data) => {
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})
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.catch((err) => {
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})
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.then((data) => {
})
.catch((err) => {
})
.finally(() => {
})
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Let's re-write our example with Promise
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getLatestBooking(userId)
.then((booking) => {
if (booking) {
return cancelBooking(booking.id)
}
})
.then((cancelledBooking) => {
if (cancelledBooking && cancelledBooking.paid) {
return refundUser(userId, cancelledBooking.paidAmount)
}
})
.then((refund) => {
if (refund) {
}
})
.catch((err) => {
console.error(err)
})
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if (booking) {
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}
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})
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}
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})
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.then((refund) => {
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if (refund) {
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}
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})
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.catch((err) => {
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console.error(err)
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})
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if (booking) {
}
})
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}
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})
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.then((refund) => {
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if (refund) {
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}
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})
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.catch((err) => {
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console.error(err)
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})
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}
})
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if (booking) {
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}
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})
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.then((refund) => {
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if (refund) {
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}
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})
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.catch((err) => {
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console.error(err)
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})
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.then((refund) => {
if (refund) {
}
})
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if (booking) {
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}
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})
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}
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})
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.catch((err) => {
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console.error(err)
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})
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.catch((err) => {
console.error(err)
})
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if (booking) {
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}
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})
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}
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})
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.then((refund) => {
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if (refund) {
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}
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})
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if (booking) {
}
})
}
})
.then((refund) => {
if (refund) {
}
})
.catch((err) => {
console.error(err)
})
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enters...
Async/Await
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Sometimes, we just want to wait for a promise to
resolve before executing the next line...
const data = await promiseObj
// ... process the data
await allows us to do exactly that
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const data = await doSomethingAsync(arg1, arg2)
We don't have to assign the promise to a variable to use await
Sometimes, we just want to wait for a promise to
resolve before executing the next line...
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try {
const data = await doSomethingAsync(arg1, arg2)
} catch (err) {
// ... handle error
}
Unified error handling
If we await a promise that eventually rejects we can capture the error with a regular try/catch block
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Async functions
async function doSomethingAsync(arg1, arg2) {
// ...
}
special keyword that marks a function as async
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Async functions
return 'SomeValue'
}
function doSomethingAsync(arg1, arg2) {
return Promise.resolve('SomeValue')
}
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Async functions
throw new Error('SomeError')
}
function doSomethingAsync(arg1, arg2) {
return Promise.reject(new Error('SomeError'))
}
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Async functions
const res1 = await doSomethingElseAsync()
const res2 = await doEvenMoreAsync(res1)
const res3 = await keepDoingStuffAsync(res2)
// ...
}
inside an async function you can use await to
suspend the execution until the awaited promise
resolves
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Async functions
const res = await doSomethingElseAsync()
if (res) {
for (const record of res1.records) {
await updateRecord(record)
}
}
}
Async functions make it very easy to write code
that manages asynchronous control flow
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Let's re-write our example with async/await
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async function cancelLatestBooking(userId) {
const booking = await getLatestBooking(userId)
if (!booking) {
return
}
await cancelBooking(booking.id)
if (booking.paid) {
await refundUser(userId, booking.paidAmount)
}
}
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}
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if (!booking) {
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return
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}
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if (booking.paid) {
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}
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if (!booking) {
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return
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}
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if (booking.paid) {
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}
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}
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if (!booking) {
return
}
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if (booking.paid) {
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}
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}
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if (!booking) {
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return
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}
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if (booking.paid) {
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}
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}
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if (!booking) {
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return
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}
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if (booking.paid) {
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}
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}
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if (booking.paid) {
}
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if (!booking) {
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return
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}
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}
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if (!booking) {
return
}
if (booking.paid) {
}
}
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Mini summary
Async/Await generally helps to keep the code simple &
readable
To use Async/Await you need to understand Promise
To use Promise you need to understand callbacks
callbacks → Promise → async/await
Don't skip any step of the async journey!
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Sequential execution
const users = ['Peach', 'Toad', 'Mario', 'Luigi']
for (const userId of users) {
await cancelLatestBooking(userId)
}
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}
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}
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}
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}
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Sequential execution (gotcha!)
users.forEach(async (userId) => {
})
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⚠Don't do this with Array.map() or Array.forEach()
Array.forEach() will run the provided function without
awaiting for the returned promise, so all the invocation will
actually happen concurrently!
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Concurrent execution (Promise.all)
await Promise.all(
users.map(
userId => cancelLatestBooking(userId)
)
)
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Promise.all() receives a list of promises and it returns a
new Promise. This promise will resolve once all the original
promises resolve, but it will reject as soon as ONE promise
rejects
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Concurrent execution (Promise.allSettled)
const results = await Promise.allSettled(
users.map(
userId => cancelLatestBooking(userId)
)
)
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[
{ status: 'fulfilled', value: true },
{ status: 'fulfilled', value: true },
{ status: 'rejected', reason: Error },
{ status: 'fulfilled', value: true }
]
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You want to use async/await but...
you have a callback-based API! 😣
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Node.js offers promise-based alternative APIs
Callback-based Promise-based
setTimeout, setImmediate, setInterval import timers from 'timers/promises'
import fs from 'fs' import fs from 'fs/promises'
import stream from 'stream' import stream from 'stream/promises'
import dns from 'dns' import dns from 'dns/promises'
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util.promisify()
import { gzip } from 'zlib' // zlib.gzip(buffer[, options], callback)
import { promisify } from 'util'
const gzipPromise = promisify(gzip)
const compressed = await gzipPromise(Buffer.from('Hello from Node.js'))
console.log(compressed) // <Buffer 1f 8b 08 00 00 00 00 ... 00 00 00>
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Promisify by hand 🖐
function gzipPromise (buffer, options) {
gzip(buffer, options, (err, gzippedData) => {
if (err) {
return reject(err)
}
resolve(gzippedData)
})
})
}
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}
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if (err) {
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return reject(err)
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}
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})
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})
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})
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if (err) {
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return reject(err)
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}
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})
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}
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})
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if (err) {
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return reject(err)
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}
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})
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}
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if (err) {
return reject(err)
}
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})
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})
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}
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if (err) {
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return reject(err)
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}
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})
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})
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}
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if (err) {
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return reject(err)
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}
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})
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})
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}
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if (err) {
return reject(err)
}
})
})
}
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loige 65

What if we we want to do the opposite? 🤷
Convert a promise-based function to a callback-based one
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var env = nunjucks.configure('views')
env.addFilter('videoTitle', function(videoId, cb) {
// ... fetch the title through youtube APIs
// ... extract the video title
// ... and call the callback with the title
}, true)
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OK, this is not a common use case, so let me give you a real
example!
Nunjucks async filters
{{ data | myCustomFilter }}
We are forced to pass a callback-based
function here!
Ex: {{ youtubeId | videoTitle }}
loige 67

util.callbackify()
import { callbackify } from 'util'
import Innertube from 'youtubei.js' // from npm
async function videoTitleFilter (videoId) {
const youtube = await new Innertube({ gl: 'US' })
const details = await youtube.getDetails(videoId)
return details.title
}
const videoTitleFilterCb = callbackify(videoTitleFilter)
videoTitleFilterCb('18y6OjdeR6o', (err, videoTitle) => {
if (err) {
console.error(err)
return
}
console.log(videoTitle)
})
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}
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if (err) {
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console.error(err)
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return
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}
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})
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}
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if (err) {
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console.error(err)
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return
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}
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})
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}
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if (err) {
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console.error(err)
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return
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}
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}
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if (err) {
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return
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})
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if (err) {
console.error(err)
return
}
})
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}
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}
if (err) {
console.error(err)
return
}
})
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19 loige 68

Callbackify by hand ✋
// ...
}
function videoTitleFilterCb (videoId, cb) {
videoTitleFilter(videoId)
.then((videoTitle) => cb(null, videoTitle))
.catch(cb)
}
// ...
})
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function videoTitleFilterCb (videoId, cb) {
videoTitleFilter(videoId)
.then((videoTitle) => cb(null, videoTitle))
.catch(cb)
}
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// ...
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}
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// ...
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})
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loige 69

OK, Cool!
But is this stuff worth it?
🧐
loige 70

Let me show you a cool performance
trick for Web Servers!
😎
loige 71

The request batching pattern
one user
/api/hotels/rome
Web server DB
loige 72

multiple users (no batching)
Web server DB
/api/hotels/rome
/api/hotels/rome
/api/hotels/rome
73
loige

multiple users (with batching!)
Web server DB
/api/hotels/rome
/api/hotels/rome
/api/hotels/rome
74
📘 Requests in-flight
/api/hotels/rome
✅
loige

The web server
import { createServer } from 'http'
const urlRegex = /^/api/hotels/([w-]+)$/
createServer(async (req, res) => {
const url = new URL(req.url, 'http://localhost')
const matches = urlRegex.exec(url.pathname)
if (!matches) {
res.writeHead(404, 'Not found')
return res.end()
}
const [_, city] = matches
const hotels = await getHotelsForCity(city)
res.writeHead(200)
res.end(JSON.stringify({ hotels }))
}).listen(8000)
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if (!matches) {
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}).listen(8000)
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}).listen(8000)
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}).listen(8000)
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}).listen(8000)
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}).listen(8000)
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}).listen(8000)
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if (!matches) {
return res.end()
}
res.writeHead(200)
}).listen(8000)
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loige 75

The data fetching function (with batching)
let pendingRequests = new Map()
function getHotelsForCity (cityId) {
if (pendingRequests.has(cityId)) {
return pendingRequests.get(cityId)
}
const asyncOperation = db.query({
text: 'SELECT * FROM hotels WHERE cityid = $1',
values: [cityId],
})
pendingRequests.set(cityId, asyncOperation)
asyncOperation.finally(() => {
pendingRequests.delete(cityId)
})
return asyncOperation
}
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20 loige 76

Benchmarks
loige.link/req-batch-bench
Without request batching With request batching (+90% avg req/sec)*
* This is an artificial benchmark and results might vary significantly in real-life scenarios. Always run your own
benchmarks before deciding whether this optimization can have a positive effect for you.
loige 77

Closing Notes
JavaScript can be a very powerful and convenient language
when we have to deal with a lot of I/O (e.g. web servers)
The async story has evolved a lot in the last 10-15 years: new
patterns and language constructs have emerged
Async/Await is probably the best way to write async code today
To use Async/Await correctly you need to understand Promise
and callbacks
Take your time and invest in learning the fundamentals
loige 78

Cover Picture by on
Marc-Olivier Jodoin Unsplash
THANKS! 🙌❤
nodejsdp.link
loige
Grab these slides!
😍Grab the book
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