如何在 switch 语句中使用 instanceof [英] How to use instanceof in a switch statement

问题描述

我使用自定义错误(es6-error)允许我处理基于他们的班级是这样的:

import { DatabaseEntryNotFoundError, NotAllowedError } from 'customError';函数 fooRoute(req, res) {doSomethingAsync().then(() => {//解决/成功返回 res.send(200);}).catch((错误) => {//拒绝/失败如果(DatabaseEntryNotFoundError 的错误实例){返回 res.send(404);} else if (error instanceof NotAllowedError) {返回 res.send(400);}log('由于未指定的错误执行异步操作失败:', error);返回 res.send(500);};}

现在我更愿意为这种类型的流使用开关，结果如下:

import { DatabaseEntryNotFoundError, NotAllowedError } from 'customError';函数 fooRoute(req, res) {doSomethingAsync().then(() => {//解决/成功返回 res.send(200);}).catch((错误) => {//拒绝/失败开关(错误实例){案例 NotAllowedError:返回 res.send(400);案例 DatabaseEntryNotFoundError:返回 res.send(404);默认:log('由于未指定的错误执行异步操作失败:', error);返回 res.send(500);}});}

instanceof 不像然而.所以后者失败了.

有没有办法在 switch 语句中检查实例的类?

解决方案

一个不错的选择是使用 constructor property 对象:

//拒绝/失败开关(错误.构造函数){案例 NotAllowedError:返回 res.send(400);案例 DatabaseEntryNotFoundError:返回 res.send(404);默认:log('由于未指定的错误执行异步操作失败:', error);返回 res.send(500);}

注意 constructor 必须与创建的对象完全匹配(假设 error 是 NotAllowedError 和 的实例NotAllowedError 是 Error) 的子类:

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

这与 instanceof 不同，它也可以匹配超类:

• NotAllowedError 的错误实例 是 true
• error instanceof Error 是 true

查看这篇有趣的帖子，了解constructor 属性.>

I use custom errors (es6-error) allowing me to handle errors based on their class like so:

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);
    };
}

Now I'd rather use a switch for this type of flow, resulting in something like:

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);
      }
    });
}

instanceof doesn't work like that however. So the latter fails.

Is there any way to check an instance for its class in a switch statement?

解决方案

A good option is to use the constructor property of the object:

// 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);
}

Notice that the constructor must match exactly with the one that object was created (suppose error is an instance of NotAllowedError and NotAllowedError is a subclass of Error):

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

This makes a difference from instanceof, which can match also the super class:

• error instanceof NotAllowedError is true
• error instanceof Error is true

Check this interesting post about constructor property.

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