TypeScript ImferredOptionType

types.ts

interface Options {
  type: "boolean" | "number" | "string" | "array";
}

type InferredOptionType<O extends Options> =
  O extends { type: "array" } ? Array<string | number> :
  O extends { type: "boolean" } ? boolean :
  O extends { type: "number" } ? number :
  O extends { type: "string" } ? string :
  unknown;

type Result<T = {}> = T & {
  [key: string]: unknown
}

class OBJ<T> {

  add<K extends string, O extends Options>(key: K, options: O) {
    return new OBJ<T & { [key in K]: InferredOptionType<O> }>()
  }

  get result() {
    return {} as { [key in keyof Result<T>]: Result<T>[key] }
  }

}

const obj = new OBJ()

obj
  .add('foo', {
    type: 'array'
  })
  .add('bar', {
    type: 'string'
  })
  .add('test', {
    type: 'number'
  })
  .result
  .bar