How to narrow a HttpsError from a Firebase Cloud Function?

Question

I have an onCall Firebase Cloud Function named getZohoDeskTicketById that throws an error like this:

throw new functions.https.HttpsError( 'unknown', error.response.data.errorCode, error.response.data );

And I am calling it like this:

import { httpsCallable, FunctionsError } from 'firebase/functions'; // ... const getZohoDeskTicketById = httpsCallable(functions, 'getZohoDeskTicketById'); const handleClick = async () => { try { const result = await getZohoDeskTicketById({ ticketId: '345112301899997', }); console.log(result); } catch (error) { if (error instanceof FunctionsError) { // TypeScript warning here console.log(error.code); // TypeScript warning here console.log(error.message); // TypeScript warning here console.log(error.details); // TypeScript warning here } } };

But I'm having trouble with narrowing down the catch.

FunctionsError has a TypeScript warning of 'FunctionsError' only refers to a type, but is being used as a value here. ts(2693)

and

error on each of my console.log's has a TypeScript warning of Object is of type 'unknown'.ts(2571)

So what is the correct way of narrowing down this error?

Does this answer your question? Why does 'instanceof' in TypeScript give me the error "'Foo' only refers to a type, but is being used as a value here."? — Dharmaraj
– Dharmaraj, Commented Mar 27, 2022 at 12:33
@Dharmaraj some of the links in that answer do explain the broader concept but I was looking for a more specific answer relating to Firebase Cloud Functions. It did help though, so thanks for linking, and I have added such an answer below. — TinyTiger
– TinyTiger, Commented Mar 27, 2022 at 22:25

TinyTiger · Accepted Answer · 2022-03-27 22:21:45Z

I created a user-defined type guard like so:

function isFunctionsError(error: unknown): error is FunctionsError { return (error as FunctionsError).details !== undefined; } const handleClick = async () => { try { const result = await getZohoDeskTicketById({ ticketId: '345112301899997', }); console.log(result); } catch (error) { if (isFunctionsError(error)) { console.log(error.code); console.log(error.message); console.log(error.details); } } };

But if anyone knows of a better way, please post it!

Yup, using type predicates seems to be the correct way of arrow to an interface: typescriptlang.org/docs/handbook/2/…

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