Hi, I have a typescript application. I want to cre...

# getting-started

[Susmitha Gudapati]

Hi, I have a typescript application. I want to create a Kotlin based application basing the same ts contracts/states. So I’ve started converting them from typescript to kotlin using custom scripts. While doing so, I’m facing a few challenges and one of them is to identity the kotlin’s equivalent implementation of typescript’s optional property, which is denoted by ? in states.

For example: My contract in ts is -

export interface A {
    id: number,
    name: string,
    imageId?: number,
    modified?: string,
}

In the interface A, the imageId and modified properties are optional. How do we denote the same functionality in kotlin’s classes?

[Umar Ata]

https://kotlinlang.org/docs/null-safety.html#safe-calls

[Susmitha Gudapati]

But this is defining that a property can be a null. So in short, nullable type. I'm looking for optional properties in an object

[Roukanken]

Sadly (thankfully?) Kotlin has no such thing as optional properties. You can emulate the functionality with one of (or multiple of):

a) nullable types
b) default arguments
c) class inheritance

based on what exactly your classes represent

[Roukanken]

and @nullable types: property?: type in interface is same as if you said property: type | undefined
so if you had a Kotlin

data class A(
    id: Int,
    name: String,
    imageId: Int? = null,
    modified: String? = null,
)

then it's constructor would behave same as constructing ts/js objects in interface A
eg, equivalent of

const a: A = {
    id: 1,
    name: "The Answer",
    imageId: 42,
}

would be

val a = A(
    id = 4,
    name = "The Answer",
    imageId = 42,
)

(and yes, modified would be null and not undefined, but unless you do some heavy magic, you won't be able to distinguish the two... but your TS code also shouldn't make difference between null & undefined)

[Susmitha Gudapati]

okay cool, thanks.