typesafeAPI.ts

type JSONValue =
  | string
  | number
  | boolean
  | null
  | JSONValue[]
  | {[key: string]: JSONValue};
type ParsedJSON = {[key: string]: JSONValue};

function assertMatchesSchema<Schema extends yup.ObjectSchema>(
  json: ParsedJSON,
  schema: Schema
): asserts json is yup.InferType<Schema> {
  try {
    schema.validateSync(json);
  } catch (error) {
    if (error instanceof yup.ValidationError) {
      throw new TypeError(
        `API response is missing fields or has invalid members. Details: ${error}`
      );
    } else {
      throw error;
    }
  }
}

async function sendAndValidateRequest<Schema extends yup.ObjectSchema>(
  url: string,
  validationSchema: Schema
): Promise<yup.InferType<Schema>> {
  const response = await (await fetch(url)).json();
  assertMatchesSchema(response, validationSchema);
  return response;
}