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Why we ditched JavaScript for Kotlin/JS? FOSDEM 2023 - JavaScript DevRoom Presented by Louis CAD, Google Developer Expert for Kotlin

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Why we ditched JavaScript for Kotlin/JS? Please answer

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Why we ditched JavaScript for Kotlin/JS? Wrong answers only • Because Kotlin is cool 😎 • Because of the hype 🏝 • Because we love to hate JavaScript • Because JavaScript was designed in just 7 days while Kotlin was in 6 years

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What can we do on the WEB with Kotlin?

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Demo of play.data2viz.io

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Demo of play.data2viz.io Forces balls + events Ripped Curtains Wind simulation 3 Forces documentation

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Demo of charts.kt on play.data2viz.io Stacked Area chart Charts: Bubble Chart Charts.kt color themes

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What can we do on the WEB with Kotlin? • A lot of complicated things • Manipulate the DOM • Draw on Canvas • Write readable and shareable business logic • Sky is the limit

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Why we ditched JavaScript for Kotlin/JS! 3 answers. 1. Safety and reliability ✅ 2. Language design ✍ • Concise syntax • Type safety • Cancellable async programming (aka. coroutines) • Many other features… 3. Code sharing 🤗 • Share what you want, native interoperability

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Why we ditched JavaScript for Kotlin/JS! 3. Code sharing 🤗 


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Let's start with #3 : Code sharing

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Let's start with #3 : Code sharing Because sharing is caring…?

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Code sharing Because sharing is caring…?

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Code sharing Which platforms does Kotlin run on?

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Code sharing Which platforms does Kotlin compile to?

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Kotlin compiles to these platforms • JVM (server, desktop, libraries…) • JavaScript (browser, nodeJS) • Android (& Wear OS) within ART (≃JVM) • Native • iOS, watchOS, macOS, tvOS • Linux • Windows • Android NDK

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Kotlin runs natively

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Kotlin runs natively on nearly all devices.

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Kotlin is not all about Kotlin.

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Interoperability! Kotlin is not all about Kotlin.

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interoperability! Kotlin is not all about Kotlin.

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Kotlin has direct interoperability

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Kotlin has direct interoperability with the following programming languages • Java • JavaScript • TypeScript (since Kotlin 1.8) • Objective-C • Swift • C

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Kotlin has direct interoperability with the following programming languages • Java (both ways) • JavaScript (both ways, some quirks) • TypeScript (both ways, some quirks) • Objective-C (both ways, some quirks) • Swift (through Objective-C, & also with dedicated community plugins) • C (both ways, some quirks)

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What direct interop means • Not reliant on third party plugins updates like in Cordova, Flutter, etc. • Use iOS, Android, JS, Java, & C APIs like they were built in Kotlin • Call new Kotlin code from Java, like it was built in Java* • Call new Kotlin code from Swift, like it was built in Swift* • Call new Kotlin code from JavaScript, like it was built in JavaScript* * limitations might apply

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That makes Kotlin the most interoperable programming language

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Why we ditched JavaScript for Kotlin/JS! 3 answers. 1. Safety and reliability ✅ 2. Language design ✍ • Concise syntax • Type safety • Cancellable async programming (aka. coroutines) • Many other features… 3. Code sharing 🤗 • Share what you want, native interoperability

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Why we ditched JavaScript for Kotlin/JS! 3 answers. 1. Safety and reliability ✅ 
 
 


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Why we ditched JavaScript for Kotlin/JS! 3 answers. 1. Safety and reliability ✅ 
 
 


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Safety and reliability

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Safety and reliability Reason number one.

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JavaScript be like Your users

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Your users Applies to other dynamic typed programming languages (Bash, Python, etc…). Missing bolts, wing off? No problem Let me try to f ly it anyway. Let me try to run it anyway. JavaScript be like

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Kotlin be like

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Kotlin be like

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No users. Bolt required 👆 No takeo ff possible. Kotlin be like

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No users. Wrong bolt type 👉 No takeo ff possible. Kotlin be like

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No users. Expected a pilot seat but got an economy seat. No takeo ff possible. Kotlin be like

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You don't have to f ly the plane to realize…

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Compiling isn't just about browsing the WEB while waiting. You don't have to f ly the plane to realize… …that one wing wasn't attached securely.

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Compiling can actually save you time! Compiled language means less tests to write

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What should we do on the WEB with Kotlin? • WEB apps • Webpages/websites with lots of dynamic logic that can't move server-side • Anything that doesn't have to be less than 300kB (i.e. landing page) • Generate html with Kotlin code (even not Kotlin/JS) • Libraries for Kotlin consumers • Complex libraries for Typescript/JavaScript consumers

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Why we ditched JavaScript for Kotlin/JS! 3 answers. 1. Safety and reliability ✅ 2. Language design ✍ • Concise syntax • Type safety • Cancellable async programming (aka. coroutines) • Many other features… 3. Code sharing 🤗 • Share what you want, native interoperability

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Why we ditched JavaScript for Kotlin/JS! 2. Language design ✍ • Concise syntax • Type safety • Cancellable async programming (aka. coroutines) • Many other features… 
 
 


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Basics

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Variables val attendees = 23

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Variables val attendees = 23 attendees++

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Variables val attendees = 23 attendees++ // compilation error Immutable reference

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Variables var attendees = 23 attendees++ Mutable reference

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Variables var attendees = 23 attendees++ // ok Mutable reference

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Functions fun welcomeMessage(name: String): String { 
 return "Welcome back " + name 
 }

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Functions fun welcomeMessage(name: String): String = "Welcome back " + name

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Functions fun welcomeMessage(name: String) = "Welcome back " + name

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Classes class User

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Classes class User()

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Classes class User(val firstName: String, val lastName: String, val middleName: String = "")

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Classes class User( val firstName: String, val lastName: String, val middleName: String = "" )

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Classes data class User( val firstName: String, val lastName: String, val middleName: String = "" )

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Classes open class User( val firstName: String, val lastName: String, val middleName: String = "" )

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Classes open class Vehicle(val name: String) 
 
 class Car(name: String) : Vehicle(name)

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Classes open class Vehicle(val name: String) 
 
 class Car(name: String) : Vehicle(name), Comparable { 
 override fun compareTo(other: Car): Int { 
 TODO() 
 } 
 }

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Classes open class Vehicle(val name: String) 
 
 class Car(name: String) : Vehicle(name), Comparable { 
 override fun compareTo(other: Car) = TODO() 
 }

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Language features

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Null safety • Nullable? types • NPE safe equals NPE: NullPointerException (similar to TypeError null/unde f ined in JS)

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Null safety var greeting: String = "Hello World!" 
 greeting = null // compilation error

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Null safety var greeting: String = "Hello World!" 
 greeting = null

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Null safety var greeting: String? = "Hello World!" 
 greeting = null

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Null safety var greeting: String? = "Hello World!" 
 greeting = null // ok

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Null safety var greeting: String? = "Hello World!" 
 greeting = null // ok 
 greeting.length // compilation error

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Null safety var greeting: String? = "Hello World!" 
 greeting.length // compilation error

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Null safety var greeting: String? = "Hello World!" 
 greeting?.length // ok Safe call operator

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Null safety var greeting: String? = "Hello World!" 
 greeting!!.length Non null assertion

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Null safety var greeting: String? = "Hello World!" 
 greeting = null greeting!!.length // compiles throws NullPointerException

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Null safety var greeting: String? = "Hello World!" 
 val chars: Int = greeting?.length // error

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Null safety var greeting: String? = "Hello World!" 
 val chars: Int = greeting?.length ?: 0 //ok Elvis operator

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Null safety assert("Bonjour" == "Bonjour")

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Null safety assert("Bonjour" == "Bonjour") assert “Bonjour".equals("Bonjour");

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Null safety assert("Bonjour" === "Bonjour") assert “Bonjour” == "Bonjour";

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Null safety nullableValue == otherNullableValue No NullPointerException!

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Properties • Similar to f ields, but include getter and setters • Can be delegated (Google for "Kotlin delegated properties")

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Properties var thing1: Something = someInitialValue 
 val thing2: Whatever = Whatever()

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Properties var thing1 = someInitialValue 
 val thing2 = Whatever()

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Properties var stringRepresentation: String 
 get() = this.toString() 
 set(value) { 
 setDataFromString(value) // Parses the string and // assigns values to other properties. 
 }

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Extension functions • Static methods (no dynamic/surprising resolution) • “instance-method like” syntax • Works even on f inal classes (default in Kotlin)

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Extension functions if (3.isEven()) println("I don't believe you!") fun Int.isEven() = this % 2 == 0

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Extension functions if (3.isEven()) println("I don't believe you!") if (9.isMultipleOf(3)) println("For sure!") fun Int.isMultipleOf(other: Int) = this % other == 0 fun Int.isEven() = isMultipleOf(2)

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Extension functions if (3.isEven()) println("I don't believe you!") if (9 isMultipleOf 3) println("For sure!") infix fun Int.isMultipleOf(other: Int) = this % other == 0 fun Int.isEven() = isMultipleOf(2)

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Extension functions inline fun Int.hasFlag(flag: Int) = flag and this == flag 
 inline fun Int.withFlag(flag: Int) = this or flag 
 inline fun Int.minusFlag(flag: Int) = this and flag.inv() if (0b1011.hasFlag(0b0001)) println("I think so!") if (0xF003.hasFlag(0x0001)) println("Yes too") if (0x0000.hasFlag(0x0001)) println("Impossible")

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Higher-order functions Kézako? A function that takes another function as a parameter (or multiple ones)

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Higher-order functions fun logEntryAndExit( name: String, block: () -> Unit ) { }

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Higher-order functions fun logEntryAndExit( name: String, block: () -> Unit ) { } logEntryAndExit( )

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Higher-order functions fun logEntryAndExit( name: String, block: () -> Unit ) { } logEntryAndExit( "Calling fancy code", )

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Higher-order functions fun logEntryAndExit( name: String, block: () -> Unit ) { } logEntryAndExit( "Calling fancy code", { fancyCode() } )

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Higher-order functions fun logEntryAndExit( name: String, block: () -> Unit ) { try { } finally { } } logEntryAndExit( "Calling fancy code", { fancyCode() } )

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Higher-order functions fun logEntryAndExit( name: String, block: () -> Unit ) { try { println("Start: $name") } finally { println("End: $name") } } logEntryAndExit( "Calling fancy code", { fancyCode() } )

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Higher-order functions fun logEntryAndExit( name: String, block: () -> Unit ) { try { println("Start: $name") block() } finally { println("End: $name") } } logEntryAndExit( "Calling fancy code", { fancyCode() } )

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Higher-order functions inline fun logEntryAndExit( name: String, block: () -> Unit ) { try { println("Start: $name") block() } finally { println("End: $name") } } logEntryAndExit( "Calling fancy code", { fancyCode() } )

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Higher-order functions inline fun logEntryAndExit( name: String, block: () -> Unit ) { try { println("Start: $name") block() } finally { println("End: $name") } } logEntryAndExit( name = "Calling fancy code", block = { fancyCode() } )

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Higher-order functions inline fun logEntryAndExit( name: String, block: () -> Unit ) { try { println("Start: $name") block() } finally { println("End: $name") } } logEntryAndExit("Calling fancy code") { fancyCode() }

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Lambdas The "anonymous" functions passed to higher-order functions • Can be inlined! Avoids unneeded memory allocation for better perfs.

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Lambdas aka. anonymous functions inline fun logEntryAndExit( name: String, block: () -> Unit ) { try { println("Start: $name") block() } finally { println("End: $name") } } logEntryAndExit("Calling fancy code") { fancyCode() }

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Lambdas aka. anonymous functions inline fun logEntryAndExit( name: String, block: () -> Unit ) { try { println("Start: $name") block() } finally { println("End: $name") } } logEntryAndExit( name = "Calling fancy code", block = { fancyCode() } )

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Lambdas val numbers = intArrayOf(1, 2, 3, 4, 5) 
 numbers.filter { it % 2 == 0 }

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when expression fun cases(obj: Any) { 
 when (obj) { 
 1 -> println("One") 
 "Hello" -> println("Greeting") 
 is Long -> println("Long") 
 !is String -> println("Not a string") 
 else -> println("Unknown") 
 } 
 }

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String templates fun greet(name: String) { 
 println("Bonjour $name!") 
 }

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String templates fun greet(name: String) { 
 println("Bonjour ${name.first().uppercase()}.!") 
 }

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Built-in singletons • Lazy instantiation • No parameter • Can extend any class

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Built-in singletons object Unit { 
 override fun toString() = "kotlin.Unit" 
 }

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Smart casts fun getStringLength(obj: Any): Int? { 
 if (obj is String) 
 return obj.length // no cast to String is needed 
 return null 
 }

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Kotlin killer feature? Besides best-in-class interop

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Structured concurrency aka. coroutines, is Kotlin's killer feature

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What are coroutines? • Saves you from callback hell • A way to write asynchronous code sequentially, without nested callbacks • Similar to async/await from JavaScript and other languages • Also Structured Concurrency

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What is Structured Concurrency? • A system to prevent concurrency issues • Requires a parent scope to launch a new, concurrent coroutine • Propagates cancellation and crashes at the right place

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How to get started? Best way to learn and leverage is to try and experiment • Open IntelliJ IDEA (Community Edition is enough) • New Project > Kotlin > Browser application • Learn Kotlin syntax and basics at kotl.in/learn • Learn about Kotlin for JavaScript at kotl.in/js • Learn about Kotlin Multiplatform at kotl.in/multiplatform • Learn about coroutines and structured concurrency at kotl.in/coroutines, with Google search, and by watching Roman Elizarov's talks

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Questions, comments, ideas… Thank you for listening. LouisCAD on GitHub (social links on my pro fi le) Blog at blog.louiscad.com (you can subscribe) Kotlin