# Enum

In NAPI-RS, Rust enum is basically transformed into a plain JavaScript Object.

#[napi]
pub enum Kind {
  Duck,
  Dog,
  Cat,
}

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export const enum Kind {
  Duck,
  Dog,
  Cat,
}

In TypeScript, numeric enum members also get a reverse mapping from enum values to enum names. However, in Rust, we don't have this reverse mapping behavior. It is just a plain JavaScript Object.

Numeric variants default to consecutive i32 values starting at zero. Explicit Rust integer discriminants are preserved, and following implicit variants continue from the previous value.

# String enum

#[napi(string_enum)]
pub enum Kind {
  Duck,
  Dog,
  Cat,
}

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export const enum Kind {
  Duck = 'Duck',
  Dog = 'Dog',
  Cat = 'Cat',
}

Use string_enum = "case" to transform every variant name, or #[napi(value = "...")] on one variant to choose its exact JavaScript value. Supported cases are listed in the #[napi] attribute reference.

#[napi(string_enum = "kebab-case")]
pub enum AccessMode {
  ReadOnly,
  #[napi(value = "read-write")]
  Writable,
}

# Structured enum

An enum with a data-carrying variant becomes a discriminated object union rather than a JavaScript enum object.

#[napi]
pub enum Event {
  Ready,
  FileChanged { path: String },
  Progress(u32, u32),
}

export type Event =
  | { type: 'Ready' }
  | { type: 'FileChanged'; path: string }
  | { type: 'Progress'; field0: number; field1: number }

The discriminator is type by default. Change it with discriminant = "kind", and transform variant values with discriminant_case = "camelCase" or another supported case. Named variant fields keep their names; tuple fields become field0, field1, and so on. A field cannot have the same JavaScript name as the discriminator.

Structured enum conversion is owned: accepting one reads and converts its fields into a Rust enum value, while returning one creates a new JavaScript object. object_from_js = false or object_to_js = false can make the type one-directional. See Type conversions.

NAPI-RS doesn't support generating Rust enum impl into JavaScript.