Typescript: conditional typecheck with instanceof not working as expected

Question

export const initPeer = (t: Chat | Diagnostic, test?: boolean) => {

  // bleep bloop ... code here

  if (test && t instanceof Diagnostic) {
    setupPeerTest(t, p);
  } else if (t instanceof Chat) {
    setupPeer(t, p);
  }
};

this code gives me the TSError. I would have thought that I already forced them to be of one type or another...

src/components/Chat/peer.ts:139:19 - error TS2345: Argument of type 'Chat | Diagnostic' is not assignable to parameter of type 'Diagnostic'.
  Type 'Chat' is missing the following properties from type 'Diagnostic': defaultState, browsers, append, isPassed

139     setupPeerTest(t, p);
                      ~

src/components/Chat/peer.ts:141:15 - error TS2345: Argument of type 'Chat | Diagnostic' is not assignable to parameter of type 'Chat'.
  Type 'Diagnostic' is missing the following properties from type 'Chat': tickTimer, tabDiv, adapter, default, and 28 more.

141     setupPeer(t, p);
                  ~

I wonder if it's something else before all this, and then this is a side-effect? Because I can't replicate it in the TypeScript playground. Can you update your question w/an MCVE? — T.J. Crowder
– T.J. Crowder, Commented Dec 19, 2018 at 17:46
what version of TS are you on? instanceof should act as a typeguard — pushkin
– pushkin, Commented Dec 19, 2018 at 18:11
Please post your definitions for your classes. Might have something to do with this though it's a bit different... — pushkin
– pushkin, Commented Dec 19, 2018 at 18:18
they do have a very similar structure so I think that link is the problem. I will post a full example if I can get some time — Joff
– Joff, Commented Dec 20, 2018 at 2:12

T.J. Crowder · Accepted Answer · 2018-12-19 17:38:15Z

2

I would have thought that I already forced them to be of one type or another...

I don't think TypeScript evaluates your logic to that degree. I think you'll need type assertions:

if (test && t instanceof Diagnostic) {
  setupPeerTest(<Diagnostic>t, p);
// -------------^^^^^^^^^^^^
} else if (t instanceof Chat) {
  setupPeer(<Chat>t, p);
// ---------^^^^^^
}

Or

if (test && t instanceof Diagnostic) {
  setupPeerTest(t as Diagnostic, p);
// --------------^^^^^^^^^^^^^^
} else if (t instanceof Chat) {
  setupPeer(t as Chat, p);
// ----------^^^^^^^^
}

Perhaps without the test variable in there it might be able to infer it, but...

answered Dec 19, 2018 at 17:38

T.J. Crowder

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2 Comments

Joff Over a year ago

it works, the weird part though is that the compiler never gave me errors until just now so it must have been something that changed in a more recent update of typescript?

T.J. Crowder Over a year ago

...except I can't replicate that in the PlayGround (including with the flag), so this may be incorrect. If I set up your situation, I don't get an error from TS.

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