Using io-ts with Firestore's timestamp data type

firestoreIoTs.ts

import {Firestore} from "@google-cloud/firestore"
import { firestore as firestoreAdmin } from 'firebase-admin'

import {foldMap} from 'fp-ts/lib/Array'
import {pipe} from 'fp-ts/lib/pipeable'
import {monoidSum} from 'fp-ts/lib/Monoid'
import {flatMap, some as exists} from 'lodash'
import * as io from 'io-ts'
import { Timestamp } from '@google-cloud/firestore'

type FirestoreTimestampStruct = {
    _seconds: number
    /**
     * Danger: I've seen this set to 0. Not sure if we need to enable this somehow.
     */
    _nanoseconds: number
  }

/**
 * Represents a Firestore Timestamp
 *
 * Credit to a similar example for a date:
 * https://github.com/gcanti/io-ts/blob/master/index.md#custom-types
 */
export const FirestoreTimestampCodec: io.Type<
  Timestamp,
  FirestoreTimestampStruct
> = new io.Type<Timestamp, FirestoreTimestampStruct>(
  'Timestamp',
  (u): u is Timestamp => u instanceof Timestamp,
  (u: unknown, c) => {
    try {
      const t = u as FirestoreTimestampStruct  
      return io.success(
        new Timestamp(t._seconds, t._nanoseconds),
      )
    } catch {
      return io.failure(u, c)
    }
  },
  (a) => ({
    _seconds: a.seconds,
    _nanoseconds: a.nanoseconds,
  }),
)

console.log(FirestoreTimestampCodec.decode({
    _seconds: 15252152,
    _nanoseconds: 0 //ignores this
}))

const DocCodec = io.type({
    timestamp: FirestoreTimestampCodec
})

export type Doc = io.TypeOf<typeof DocCodec>

const firestore = new Firestore({
    projectId: 'yourproject'
})

// A document with reference a and a property named timestamp is set to some typestamp
firestore.collection("test").doc("a").get().then((doc) => {
    console.log(DocCodec.decode(doc.data()))

    //Setting document update typechecks
    const newData: Doc = {
        timestamp: firestoreAdmin.Timestamp.now()
    }

    doc.ref.set(newData).then((writeRes) => console.log(writeRes), console.error)
})