Is there a way to distinguish between objects of different interface types in Javascript/TypeScript?

Question

I'm working with a library which defines the following interfaces:

LocalUser {
  data {
    value: LocalDataValue
  },
  ...various other methods etc...
}
RemoteUser {
  data {
    value: RemoteDataValue
  },
  ...various other methods etc...
}

Then it gives me a User which is defined like this (I can work out which type of user it is based on how it was obtained, but there doesn't seem to be a way to actually pass that information on):

User {
  value: LocalUser|RemoteUser
}

because for the vast majority of my code it doesn't matter. However, at one point I need to know whether I'm dealing with a LocalUser, because I only want to do a particular operation on LocalUsers. Is there a way to know whether a LocalUser is a RemoteUser or a LocalUser?

I can't find anywhere in the documentation which tells me what the difference is between them, though I know that some methods only appear on LocalUser and not on RemoteUser (eg I can do localUser.enable() but remoteUser.enable() doesn't work).

Is there a way to do something like this:

if(myUnknownUser isInstanceOf (LocalUser)) { doSomething() }

or a better way to handle this?

captain-yossarian from Ukraine · Accepted Answer · 2021-08-13 14:30:45Z

2

For such cases you should use typeguards

Consider this example:


type LocalUser = {
  tag: 'LocalUser';
  name: 'John'
}

type RemoteUser = {
  tag: 'RemoteUser';
  name: 'David'
}
type User = LocalUser | RemoteUser

// typeguard
const isLocal = (user: User): user is LocalUser => user.tag === 'LocalUser'

// typeguard
const isRemote = (user: User): user is RemoteUser => user.tag === 'RemoteUser'

declare var user: User;

if (isLocal(user)) {
  const x = user // LocalUser
}

if (isRemote(user)) {
  const x = user // RemoteUser
}

It is up to you how you will implement typeguard

answered Aug 13, 2021 at 14:30

captain-yossarian from Ukraine

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1 Comment

Sharon Over a year ago

Thank you - I hadn't heard of typeguards, but I'll go and do some research now. Thanks!

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