Last time, we talked about modeling the Schema.org class hierarchy in TypeScript. We ended up with an elegant, recursive solution that treats any type Thing as a "@type"-discriminated union of ThingLeaf and all the direct sub-classes of the type. The next challenge in the journey of building TypeScript typings for the Schema.org vocabulary is modeling Enumerations.

Learning from Examples

Let’s look at a few examples from the Schema.org website to get a better sense of what Enumerations look like.

First up, I looked at PaymentStatusType, which can take any one of these values: PaymentAutomaticallyApplied, PaymentComplete, PaymentDeclined, PaymentDue, or PaymentPastDue. PaymentStatusType is used in the paymentStatus property on the Invoice class.

Here’s an excerpt from an example of an invoice:

{
    "@context": "http://schema.org/",
    "@type": "Invoice",
    // ...
    "paymentStatus": "http://schema.org/PaymentComplete",
    "referencesOrder": [
      // ...
    ]
}

Here, the value of an Enumeration appears as an absolute IRI.

Looking at other examples, however, such as GamePlayMode which appears in playMode on VideoGame shows up differently:

{
  "@context": "http://schema.org",
  "@type": "VideoGame",
  // ...
  "playMode": "SinglePlayer",
  // ...
}

Here, SinglePlayer shows up as an IRI reference relative to the current "@context". The exact mechanics of "@context" are a bit cumbersome to describe inline here, but I’ll be discussing them in a future post. Suffice to say, in this example, if your context is "http://schema.org" then you can use "SinglePlayer" in contexts where an IRI reference is expected as a relative IRI reference, resolving to "http://schema.org/SinglePlayer".

Therefore, the same enumeration value can be represented by multiple strings that happen to resolve to the same thing. But, for us to know the set of all possible strings we can use to represent an enumeration value, we’ll need to know the @context used. As a matter of fact, this also applies to the "@type" property, so we’ve been assuming a context of http[s]://schema.org all along.

The schema-dts uses a smarter approach: when using the schema-dts-gen CLI tool, the user can specify exactly what their @context should be. Then, the "@type"s produced are rendered as the appropriate relative IRI string literals, if the context allows it, and otherwise an absolute IRI. The pre-packaged schema-dts package, uses "https://schema.org" as its context.

Choosing our Representation

Say the goal of the typings is to easily allow writing new JSON-LD. Then, clarity might be desirable: choose to only allow absolute IRIs or only allow relative IRIs (when permitted). This way, redundant but still valid JSON-LD would be rejected by the type checker, but completions would be much cleaner and all new code would look the same.

On the other hand, say the goal of the typings is to easily validate existing JSON-LD. Then, we probably want it to only emit type errors on invalid string literals. That is, we’ll probably want our typings to include all plausible ways of referencing an enum value, given a @context.

For now, we’ll choose to standardize on just absolute IRIs (like http://schema.org/SinglePlayer) since those are the most common and the clearest.

So far, one might think of representing enums simply:

enum GamePlayMode {
  SinglePlayer = "https://schema.org/SinglePlayer",
  MultiPlayer = "https://schema.org/MultiPlayer",
  CoOp = "https://schema.org/CoOp"
}

or perhaps:

type GamePlayMode =
  "https://schema.org/SinglePlayer" |
  "https://schema.org/MultiPlayer" |
  "https://schema.org/CoOp";

both of these are equally valid, though using const enums in the first approach might more desirable.

Enumerations can have other values…

If only it were that simple. For example, take DeliveryMethod which, in this example is represented as so:

  "deliveryMethod": {
    "@type": "DeliveryMethod",
    "name": "http://purl.org/goodrelations/v1#UPS"
  },

That’s right. DeliveryMethod still extends Enumeration, which is a class type. Enumeration itself extends Intangible which extends Thing.

Our representation will need to change (accounting to the insights learned from the class hierarchy article), then:

const enum GamePlayModeEnum {
  SinglePlayer = "https://schema.org/SinglePlayer",
  MultiPlayer = "https://schema.org/MultiPlayer",
  CoOp = "https://schema.org/CoOp"
}

type GamePlayModeBase = EnumerationBase;  // Doesn't extend other properties

interface GamePlayModeLeaf extends GamePlayModeBase  {
  "@type": "GamePlayMode";
}

type GamePlayMode = GamePlayModeEnum | GamePlayModeLeaf;

Here, we allow for a GamePlayMode to be either the Enum value (represented either as a union of string literals or a const enum) or the Leaf value.

Enums can have sub-classes

The DeliveryMethod example also shows us sub-types are possible. Note that ParcelService, for instance, is a class type (albeit not an interesting one; it adds no properties).

This means we’ll need to include sub-classes in our final type union of the enum, just as we did with Classes in general.

Enums can extend regular classes

A regular class can have sub-classes that themselves are enumerations. Let’s take a look at the PhysicalExam, for an example. PhysicalExam is an enum type that extends a non-enum MedicalProcedure (notice how Schema.org models that as having two parents: MedicalEnumeration and MedicalProcedure).

Using the recursive method defined previously, this means that:

  1. MedicalProcedure is a union of it’s “leaf” class, and all it’s subclasses, which includes “Enums”
  2. PhysicalExam and other enum sub-classes are defined as above: a union of a true Enum type, a leaf type, and all direct sub-classes.

Putting it all Together

Essentially, we can see that Enumerations really are just classes that can also take a special value (an IRI reference). All of the rules of classes we have learned before still apply, but now there’s an addition: our final union contains an XyzEnum type in the union, in addition to XyzLeaf and all sub-classes.

Take a look at the Audience enum for example, and notice it’s very nested nature:

Schema.org Audience has a very nested nature, with 4 direct subclasses, one of which is an enum, with subclasses of it's own.
Schema.org Audience and it’s subclasses. MedicalAudience is an Enum subtype of Audience, with a subtype of it’s own: Patient.

The rules provided so far compose nicely to allow us to represent this complex structure. I’ll leave this as an exercise to the reader to see how the TypeScript structure ends up here, but isn’t too different from the final result you’ll see in schema-dts.

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