How to use instanceof to class with private constructor

[YutaUra]

first, I want to use zod with firebase/firestore. And there is FieldValue class use for createdAt or updatedAt.

So, I think like this to use zod with firebase

import * as z from 'zod'
import firebase from 'firebase/app'

const ServerTimestampSchema = z.instanceof(firebase.firestore.FieldValue)

const UserSchema = z.object({
  name: z.string(),
  // ...
  createdAt: ServerTimestampSchema,
  updatedAt: ServerTimestampSchema
})

maybe, this works at run time, but typescript compiler throw this message.

// ...
const ServerTimestampSchema = z.instanceof(firebase.firestore.FieldValue)
// Argument of type 'typeof FieldValue' is not assignable to parameter of type 'new (...args: any[]) => any'.
//  Cannot assign a 'private' constructor type to a 'public' constructor type.ts(2345)

// ...

If someone has any solutions, please tell me!!

[colinhacks]

Hm, interesting problem. Based on a quick review, it doesn't appear to be solvable inside of instanceof. The only way for Zod to infer the type of the class passed into z.instanceof is to constrain it to the form new (...args: any[]) => any and use the InstanceType utility: https://www.typescriptlang.org/docs/handbook/utility-types.html#instancetypetype.

However, in your particular case you can easily build your own using z.custom, which was introduced specifically for things like this. It accepts an explicit type and lets you specify a custom refinement check (in this case, an instanceof check). Here's the code:

const ServerTimestampSchema = z.custom<firebase.firestore.FieldValue>(
  (data) => data instanceof firebase.firestore.FieldValue
);

[ScreamZ]

For those like me trying to validate a luxon Duration this « hack » is valid.

import { z } from "zod";
import { Duration } from "luxon";

const IntervalValidator = z.custom<Duration>((t) => t instanceof Duration);

[sweetcoco]

Here are schemas for common Firestore FieldValue and Timestamp cases:

import { FieldValue, Timestamp } from "firebase-admin/firestore";
import { z } from "zod";

// this is a Timestamp from the database
export const TimestampSchema = z.custom<Timestamp>(
  (data) => data instanceof Timestamp
);

// this is a FieldValue instance, to be written to the database
export const FieldValueSchema = z.custom<FieldValue>(
  (data) => data instanceof FieldValue
);

// this is a FieldValue, specifcally ServerTimestampTransform, to be written to the database
export const FieldValueServerTimestampSchema = z.custom<any>(
  (data) => (data as Object).constructor.name === FieldValue.serverTimestamp().constructor.name
);