Enumerated Types

OpenAPI allows us to define enumeration types, allowing us to narrow primitives like string from being any string, to a limited set of possible values. For example:

type: string
enum:
  - Apple
  - Banana
  - Orange

Defines an enumerated value that can be one of Apple, Banana, Orange

This is great for documenting the domain of valid values, but can cause problems with safely evolving your API over time, as adding a new value to the enum may become a breaking API change.

The rest of this page explores how the code generator mitigates this issue, and how you can customise the behavior if your needs are different.

Open vs Closed enumerations

We can consider an enum “open” if the parsing of it can allow for new/unknown values, and “closed” if new values should constitute a parsing error.

The OpenAPI and JSON Schema specifications don’t explicitly distinguish between open and closed enums. By default, they assume enums are closed—meaning only listed values are valid. This poses an issue for safely evolving your API surface as your needs change without it being a breaking change.

If you have exact control over all your API clients you could mitigate this by first updating the clients to support the new value, then updating the server to produce it.

However, in most real world cases this is either difficult, or not possible. A good example is native mobile applications, as there is generally a long tail of outdated app versions in the wild, and as a developer you have little control over when your users update their apps.

To prevent this issue, ideally:

• Our servers will use closed enums, and therefore only ever accept/return valid enum values
• Our clients will use open enums, and gracefully handle unrecognized enum values (likely by ignoring them, or the entity that contains them)

Code generation of enums

With that in mind, the generator takes a conservative approach for servers (closed enums) and a forward-compatible approach for clients (open enums).

Using the previous example, lets explore how this gets generated.

type: string
enum:
  - Apple
  - Banana
  - Orange

Server code (closed enum)

For server templates, we just generate the exact enum values, meaning that an error will be raised both if a client sends us an unknown value, or the server attempts to respond with one.

export type t_Fruit = "Apple" | "Banana" | "Orange"
export const s_Fruit = z.enum(["Apple", "Banana", "Orange"])

Client code (open enum)

For the client templates, we use a technique called “branded types” to include the string / number type in our union types, in such a way that typescript knows we could receive any value, but won’t let us accidentally assign an unknown value.

This works because a “branded type” creates a distinct type that TypeScript tracks separately, preventing accidental assignment of arbitrary strings at compile time, while still allowing unknown values to pass through parsing safely at runtime.

export type UnknownEnumStringValue = string & {
  _brand: "unknown enum string value"
}
 
 
export type t_Fruit = "Apple" | "Banana" | "Orange" | UnknownEnumStringValue
export const s_Fruit = z.union([
  z.enum(["Apple", "Banana", "Orange"]),
  z.string().transform((it) => it as typeof it & UnknownEnumStringValue),
])

This prevents invalid/random values being referenced in the code, whilst also allowing us to make exhaustiveness checks.

function processFruit(result: t_Fruit): void {
  switch (result) {
    case "Apple":
      console.log("bite into apple")
      break
    case "Banana":
      console.log("slip over banana")
      break
    case "Orange":
      console.log("juice orange")
      break
    default: {
      // This checks that we have exhaustively handled the known values
      const _ = result satisfies UnknownEnumStringValue
      console.warn(`unsupported ${result}, skipping`)
    }
  }
}

Whilst technically t_Fruit can be any string value at runtime, you still won’t be able to assign random values to it, as the branded type will not allow you.

This is interesting as it means that our server can start returning new enumerated values, before the clients have been updated to explicitly handle them, and it nudges developers to handle the unknown case gracefully.

Customizing the behavior

There are two ways you can customize this behavior

• Global CLI option --enum-extensibility <value>
• Per schema extension property x-enum-extensibility: <value>, overriding the global configuration

Where <value> is either open or closed.

Example:

type: string
x-enum-extensibility: closed
enum:
  - Dog
  - Cat

Union types vs Enum types

Currently all enum handling for our typescript templates use union types , rather than actual enum  statements.

This is mostly a stylistic choice, based on the authors personal preferences and subjective opinions of ergonomics. It’s possible that an option to output “real” enums may be added in the future.