如何在switch语句中使用instanceof

Question

我使用自定义错误(es6-error)允许我根据他们的类处理错误,如下所示:

import { DatabaseEntryNotFoundError, NotAllowedError } from 'customError';

function fooRoute(req, res) {
  doSomethingAsync()
    .then(() => {
      // on resolve / success
      return res.send(200);
    })
    .catch((error) => {
      // on reject / failure
      if (error instanceof DatabaseEntryNotFoundError) {
        return res.send(404);
      } else if (error instanceof NotAllowedError) {
        return res.send(400);
      }
      log('Failed to do something async with an unspecified error: ', error);
      return res.send(500);
    };
}

现在我宁愿为这种类型的流使用开关,导致类似于:

import { DatabaseEntryNotFoundError, NotAllowedError } from 'customError';

function fooRoute(req, res) {
  doSomethingAsync()
    .then(() => {
      // on resolve / success
      return res.send(200);
    })
    .catch((error) => {
      // on reject / failure
      switch (error instanceof) {
        case NotAllowedError:
          return res.send(400);
        case DatabaseEntryNotFoundError:
          return res.send(404);
        default:
          log('Failed to do something async with an unspecified error: ', error);
          return res.send(500);
      }
    });
}

然而,instanceof并不像那样工作.所以后者失败了.

有没有办法在switch语句中检查其类的实例？

Answer 1

一个好的选择是使用对象的constructor 属性:

// on reject / failure
switch (error.constructor) {
    case NotAllowedError:
        return res.send(400);
    case DatabaseEntryNotFoundError:
        return res.send(404);
    default:
        log('Failed to do something async with an unspecified error: ', error);
        return res.send(500);
}

请注意,constructor必须与创建对象的内容完全匹配(假设error是实例,NotAllowedError并且NotAllowedError是其子类Error):

• error.constructor === NotAllowedError 是 true
• error.constructor === Error 是 false

这有所不同instanceof,它也可以匹配超类:

• error instanceof NotAllowedError 是 true
• error instanceof Error 是 true

查看这篇关于constructor财产的有趣帖子.

Answer 2

解决方法，以避免if-else。在这里找到

switch (true) {
    case error instanceof NotAllowedError: 
        return res.send(400);

    case error instanceof DatabaseEntryNotFoundError: 
        return res.send(404);

    default:
        log('Failed to do something async with an unspecified error: ', error);
        return res.send(500);
}