AlexTes AlexTes - 7 months ago 41
Javascript Question

How to use instanceof in a switch statement

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

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.