default-values.md

Default values

Some properties of a class are optional, or nullable, or both.

Default values

If a value has a default value, then it is not required for creating an encoded message. Therefore, it will be marked as optional using the ?: notation.

@Serializable
class Colour(val rgb: Int = 12345)

fun main() {
  val tsGenerator = KxsTsGenerator()
  println(tsGenerator.generate(Colour.serializer()))
}

You can get the full code here.

export interface Colour {
  rgb?: number;
}

Nullable values

Properties might be required, but the value can be nullable. In TypeScript that is represented with a type union that includes null.

@Serializable
class Colour(val rgb: Int?) // 'rgb' is required, but the value can be null

fun main() {
  val tsGenerator = KxsTsGenerator()
  println(tsGenerator.generate(Colour.serializer()))
}

You can get the full code here.

export interface Colour {
  rgb: number | null;
}

Default and nullable

A property can be both nullable and optional, which gives four possible options.

@Serializable
data class ContactDetails(
  // nullable: ❌, optional: ❌
  val name: String,
  // nullable: ✅, optional: ❌
  val email: String?,
  // nullable: ❌, optional: ✅
  val active: Boolean = true,
  // nullable: ✅, optional: ✅
  val phoneNumber: String? = null,
)

fun main() {
  val tsGenerator = KxsTsGenerator()
  println(tsGenerator.generate(ContactDetails.serializer()))
}

You can get the full code here.

export interface ContactDetails {
  name: string;
  email: string | null;
  active?: boolean;
  phoneNumber?: string | null;
}

Override optional properties

Properties with default values can be set as required using the Kotlinx Serialization annotation, @kotlinx.serialization.Required .

For demonstration purposes, let's see what happens when @Required is added to all properties.

@Serializable
data class ContactDetails(
  @Required
  val name: String,
  @Required
  val email: String?,
  @Required
  val active: Boolean = true,
  @Required
  val phoneNumber: String? = null,
)

fun main() {
  val tsGenerator = KxsTsGenerator()
  println(tsGenerator.generate(ContactDetails.serializer()))
}

You can get the full code here.

active and phoneNumber are now required properties. Note that @Required had no effect on name or email; because they do not have default values, they were already required.

export interface ContactDetails {
  name: string;
  email: string | null;
  active: boolean;
  phoneNumber: string | null;
}