Javascript 如何在 switch 语句中使用 instanceof

How to use instanceof in a switch statement

提问by alextes

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

我使用自定义错误 ( es6-error) 允许我根据它们的类处理错误，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);
    };
}

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

现在我宁愿为这种类型的流程使用开关，结果如下：

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);
      }
    });
}

instanceofdoesn't work like that however. So the latter fails.

但是，instanceof不是这样工作的。所以后者失败了。

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

有没有办法在 switch 语句中检查其类的实例？

回答by Dmitri Pavlutin

A good option is to use the constructorpropertyof the object:

一个不错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);
}

Notice that the constructormust match exactly with the one that object was created (suppose erroris an instance of NotAllowedErrorand NotAllowedErroris a subclass of Error):

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

• error.constructor === NotAllowedErroris true
• error.constructor === Erroris false

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

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

这与 不同instanceof，它也可以匹配超类：

• error instanceof NotAllowedErroris true
• error instanceof Erroris true

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

Check this interesting postabout constructorproperty.

查看这篇关于constructor财产的有趣帖子。

回答by ya_dimon

Workaround, to avoid if-else. Found here

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);
}

回答by Lachlan Young

An alternative to this switch case is to just have a status field in the Error's constructor.

这种 switch case 的替代方法是在 Error 的构造函数中只有一个 status 字段。

For Example, build your error like so:

例如，像这样构建你的错误：

class NotAllowedError extends Error {
    constructor(message, status) {
        super(message);
        this.message = message;
        this.status = 403; // Forbidden error code
    }
}

Handle your error like so:

像这样处理你的错误：

.catch((error) => {
  res.send(error.status);
});