多级防护装置

Question

给定以下代码，如何修改isAppErrorOfKind类型保护，以便根据指定的kind返回类型为AppErrorHttp或AppErrorNetwork？

type AppError = AppErrorHttp | AppErrorNetwork | AppErrorUnsupported;

interface AppErrorHttp {
  kind: 'http';
  status: number;
}

interface AppErrorNetwork {
  kind: 'network';
  reason: string;
}

interface AppErrorUnsupported {
  kind: 'unsupported';
  api: string;
}

function isAppError(error: Error | AppError): error is AppError {
  if (error === undefined || error === null) return false;
  if (typeof(error) !== 'object') return false;
  return 'kind' in error;
}

function isAppErrorOfKind(error: Error | AppError, kind: AppError['kind']): error is XXX {
  if (!isAppError(error)) return false;
  return error.kind === kind;
}

Answer 1

您需要推断kind参数：

type AppError = AppErrorHttp | AppErrorNetwork | AppErrorUnsupported;

interface AppErrorHttp {
  kind: 'http';
  status: number;
}

interface AppErrorNetwork {
  kind: 'network';
  reason: string;
}

interface AppErrorUnsupported {
  kind: 'unsupported';
  api: string;
}


function isAppError(error: Error | AppError): error is AppError {
  if (error === undefined || error === null) return false;
  if (typeof error !== 'object') return false;
  return 'kind' in error;
}

function isAppErrorOfKind<Kind extends AppError['kind']>(
  error: Error | AppError,
  kind: Kind
): error is Extract<AppError, { kind: Kind }> {
  if (!isAppError(error)) return false;
  return error.kind === kind;
}

const foo = (arg: AppError) => {
  if (isAppErrorOfKind(arg, 'network')) {
    arg // AppErrorHttp
  } else {
    arg // AppErrorHttp | AppErrorUnsupported
  }
}

游乐场

Answer 2

如果我正确理解了您的问题，我可能会使用泛型类型约束：

function isAppErrorOfKind<T extends AppError>(error: T, kind: AppError["kind"]): error is T {
  if (!isAppError(error)) return false;
  return error.kind === kind;
}

然后只能传入AppError类型的错误，扩展AppErrorHttp和AppErrorNetwork。