How to use instanceof in a switch statement

Question

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?

Answer 1

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.

Answer 2

Workaround, to avoid if-else. Found here

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); }