How to use instanceof in a switch statement
JavascriptError HandlingEcmascript 6Javascript Problem Overview
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?
Javascript Solutions
Solution 1 - Javascript
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
istrue
error.constructor === Error
isfalse
This makes a difference from instanceof
, which can match also the super class:
error instanceof NotAllowedError
istrue
error instanceof Error
istrue
Check this interesting post about constructor
property.
Solution 2 - Javascript
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);
}
Solution 3 - Javascript
An alternative to this switch case is to just have a status field in the Error's constructor.
For Example, build your error like so:
class NotAllowedError extends Error {
constructor(message, status) {
super(message);
this.message = message;
this.status = 403; // Forbidden error code
}
}
Handle your error like so:
.catch((error) => {
res.send(error.status);
});