How to use instanceof in a switch statement
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);
}
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.
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);
});