[FIXED] How to use instanceof in a switch statement

Issue

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?

Solution

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.

Answered By - Dmitri Pavlutin

Answer Checked By - Clifford M. (PHPFixing Volunteer)