objectToSchema.ts

export type ObjectToSchema<TObject, TRequired extends string[] | null = null> = TObject extends readonly [infer K]
  ? ValueToProperty<[K]>
  : keyof TObject extends never
    ? { type: 'object' }
    : {
      [P in keyof TObject]: TObject[P] extends infer Value
        ? ValueToProperty<Value>
        : never
    } extends infer Properties
      ? TRequired extends null
        ? { type: 'object',
          properties: Properties }
        : { type: 'object',
          required: TRequired,
          properties: Properties }
      : never

export type ValueToProperty<TValue> = TValue extends `$${infer Ref}`
  ? { $ref: Ref } : TValue extends string
    ? { type: 'string' } : TValue extends number
      ? { type: 'number' } : TValue extends boolean
        ? { type: 'boolean' } : TValue extends new ()=> Date
          ? { type: 'string',
            format: 'date' } : TValue extends readonly [infer K]
            ? { type: 'array',
              items: ValueToProperty<K> } : TValue extends (infer K)[]
              ? { enum: K } : TValue extends Record<string, any>
                ? keyof TValue extends never
                  ? { type: 'object' }
                  : { type: 'object' } & ObjectToSchema<TValue> : never

const mapValueToProperty = (value: any): any => {
  if( value.constructor === Object ) {
    return Object.assign({
      type: 'object',
    }, fromLiteral(value))
  }

  if( value === Date ) {
    return {
      type: 'string',
      format: 'date-time',
    }
  }

  if( Array.isArray(value) ) {
    return {
      type: 'array',
      items: mapValueToProperty(value[0]),
    }
  }

  if( value && typeof value === 'string' ) {
    return {
      $ref: value,
    }
  }

  return {
    type: typeof value,
  }
}

export const fromLiteral = <
  const TObject,
  TRequired extends (keyof TObject & string)[],
>(object: TObject, required?: TRequired) => {
  const entries: [string, Property][] = []
  for( const propName in object ) {
    const value: any = object[propName]
    if( value === null || value === undefined ) {
      continue
    }

    entries.push([
      propName,
      mapValueToProperty(value),
    ])
  }

  const properties = Object.fromEntries(entries)
  return {
    type: 'object',
    required,
    properties,
  } as ObjectToSchema<TObject, TRequired>
}