Hand made inference of a Typescript type from a custom json schema

infer-ts-schema-from-json.ts

type TypeFromString<T extends string> =
  T extends "number" ? number
  : T extends "string" ? string
  : ["Invalid type string", T]

type InferPropRequired<PropSpec> = PropSpec extends { isRequired: false } ? false : true
type InferPropType<PropSpec> =
  PropSpec extends { values: ReadonlyArray<infer Values> } ? Values
  : PropSpec extends { type: infer T extends string } ? TypeFromString<T>
  : ["Invalid prop spec", PropSpec]

type InferPayload<Payload> = {
  [Prop in keyof Payload as (InferPropRequired<Payload[Prop]> extends true ? Prop : never)]: InferPropType<Payload[Prop]>
} & {
  [Prop in keyof Payload as (InferPropRequired<Payload[Prop]> extends false ? Prop : never)]+?: InferPropType<Payload[Prop]>
}

type InferSchema<Schema> = {
  [Event in keyof Schema]: Schema[Event] extends { payload: infer P } ? InferPayload<P> : ["Invalid payload for event", Event]
}

//

// Simulate `import json from "schema.json" as const`
// See https://github.com/microsoft/TypeScript/issues/32063
const json = {
  "some_metric": {
    "description": "Some event",
    "payload": {
      "index": {
        "description": "Some index",
        "type": "number"
      },
      "card_type": {
        "description": "Card type",
        "type": "number",
        "values": [0, 1]
      },
      "phrase": {
        "description": "Some phrase",
        "type": "string",
        "isRequired": false
      },
    }
  },
  "single_prop": {
    "payload": {
      "prop": {
        "type": "number"
      }
    }
  },
  "optional": {
    "payload": {
      "prop": {
        "type": "number",
        "isRequired": false,
      }
    }
  },
  "union": {
    "payload": {
      "prop": {
        "type": "number",
        "values": [17, 23]
      }
    }
  }
} as const

type Metrics = InferSchema<typeof json>
function sendMetric<Event extends keyof Metrics>(name: Event, payload: Metrics[Event]) {
  // ???
}

sendMetric("some_metric", { index: 42, card_type: 0, phrase: "asdf" })
sendMetric("some_metric", { index: 42, card_type: 1 })

sendMetric(
  //@ts-expect-error "unknown_name" is not assignable to [some of known names]
  "unknown_name",
  {}
)
sendMetric("single_prop",
  {
    //@ts-expect-error "string" is not assignable to "number"
    prop: "not-a-number",
  }
)
sendMetric("single_prop",
  //@ts-expect-error "prop" is missing
  {}
)
sendMetric("single_prop",
  {
    prop: 42,
    //@ts-expect-error may only specify known props
    unknown_prop: 42,
  }
)

sendMetric("optional", {})

sendMetric("union", { prop: 17 })
sendMetric("union", {
  //@ts-expect-error 42 is not assignable to [some of values]
  prop: 42
})