Alternative approach with replaacer and reviver methods.

dev/src/timestamp.ts

@@ -14,11 +14,10 @@
  * limitations under the License.
  */
 
-import * as firestore from '@google-cloud/firestore';
-
-import {google} from '../protos/firestore_v1_proto_api';
-import {validateInteger} from './validate';
+import * as firestore from "@google-cloud/firestore";
 
+import { google } from "../protos/firestore_v1_proto_api";
+import { validateInteger } from "./validate";
 import api = google.firestore.v1;
 
 /*!
@@ -309,10 +308,23 @@ export class Timestamp implements firestore.Timestamp {
     return formattedSeconds + '.' + formattedNanoseconds;
   }
 
-  /**
-   * Returns a value to use when encoding this object into JSON.
-   */
-  toJSON(): string {
-    return this.toDate().toISOString();
+  static reviver(this: any, key: string, value: any): any {
+    if (key === '') {
+      const match = value.match(/(\d{4}-[01]\d-[0-3]\dT[0-2]\d:[0-5]\d:[0-5]\d)\.(\d+)([+-][0-2]\d:[0-5]\d|Z)/);
+      return new Timestamp(
+        Math.floor(Date.parse(match[1] + 'Z') / 1000),
+        Number(match[2].padEnd(9, '0'))
+      );
+    }
+  }
+
+  static replacer(this: any, key: string, value: any): any {
+    if (key === '') {
+      const t: Timestamp = value;
+      const isoDateSeconds: string = new Date(t._seconds * 1000)
+        .toISOString()
+        .split('.')[0];
+      return `${isoDateSeconds}.${String(t._nanoseconds).padStart(9, '0')}Z`;
+    }
   }
 }

dev/src/write-batch.ts

@@ -96,6 +96,23 @@ export class WriteResult implements firestore.WriteResult {
         this._writeTime.isEqual(other._writeTime))
     );
   }
+
+  static reviver(this: any, key: string, value: any): any {
+    if (key === 'writeTime') {
+      return JSON.parse(value, Timestamp.reviver);
+    }
+  }
+
+  static replacer(this: any, key: string, value: any): any {
+    switch (key) {
+      case '':
+        return {
+          writeTime: value._writeTime,
+        };
+      case 'writeTime':
+        return JSON.stringify(value, Timestamp.replacer);
+    }
+  }
 }
 
 /**

dev/test/timestamp.ts

@@ -12,19 +12,14 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-import {describe, it} from 'mocha';
-import {expect} from 'chai';
-import * as through2 from 'through2';
+import { describe, it } from "mocha";
+import { expect } from "chai";
+import * as through2 from "through2";
 
-import {google} from '../protos/firestore_v1_proto_api';
-
-import * as Firestore from '../src/index';
-import {
-  ApiOverride,
-  createInstance as createInstanceHelper,
-  document,
-} from '../test/util/helpers';
+import { google } from "../protos/firestore_v1_proto_api";
 
+import * as Firestore from "../src/index";
+import { ApiOverride, createInstance as createInstanceHelper, document } from "../test/util/helpers";
 import api = google.firestore.v1;
 
 function createInstance(opts: {}, document: api.IDocument) {
@@ -54,7 +49,7 @@ const DOCUMENT_WITH_EMPTY_TIMESTAMP = document('documentId', 'moonLanding', {
   timestampValue: {},
 });
 
-describe('timestamps', () => {
+describe.only('timestamps', () => {
   it('returned by default', () => {
     return createInstance({}, DOCUMENT_WITH_TIMESTAMP).then(firestore => {
       const expected = new Firestore.Timestamp(-14182920, 123000123);
@@ -224,16 +219,39 @@ describe('timestamps', () => {
 
   it('JSON.stringify() on the smallest Timestamp object', () => {
     const timestamp = new Firestore.Timestamp(-62135596800, 0);
-    expect(JSON.stringify(timestamp)).to.equal('"0001-01-01T00:00:00.000Z"');
+    const writeResult = new Firestore.WriteResult(timestamp);
+    expect(JSON.stringify(timestamp, Firestore.Timestamp.replacer)).to.equal('"0001-01-01T00:00:00.000000000Z"');
+    expect(JSON.stringify(writeResult, Firestore.WriteResult.replacer)).to.equal('{"writeTime":"\\"0001-01-01T00:00:00.000000000Z\\""}');
+  });
+
+  it('JSON.parse() on the smallest Timestamp object', () => {
+    const timestamp = new Firestore.Timestamp(-62135596800, 0);
+    expect(JSON.parse('"0001-01-01T00:00:00.000000000Z"', Firestore.Timestamp.reviver)).to.deep.equal(timestamp);
   });
 
   it('JSON.stringify() on the largest Timestamp object', () => {
     const timestamp = new Firestore.Timestamp(253402300799, 999999999);
-    expect(JSON.stringify(timestamp)).to.equal('"9999-12-31T23:59:59.999Z"');
+    expect(JSON.parse('"9999-12-31T23:59:59.999999999Z"', Firestore.Timestamp.reviver)).to.deep.equal(timestamp);
+  });
+
+  it('JSON.parse() on the largest Timestamp object', () => {
+    const timestamp = new Firestore.Timestamp(253402300799, 999999999);
+    expect(JSON.stringify(timestamp, Firestore.Timestamp.replacer)).to.equal('"9999-12-31T23:59:59.999999999Z"');
   });
 
   it('JSON.stringify() on a Timestamp object whose date is in 2023', () => {
     const timestamp = new Firestore.Timestamp(1680272000, 840000000);
-    expect(JSON.stringify(timestamp)).to.equal('"2023-03-31T14:13:20.840Z"');
+    expect(JSON.parse('"2023-03-31T14:13:20.840000000Z"', Firestore.Timestamp.reviver)).to.deep.equal(timestamp);
+  });
+
+  it('JSON.parse() on a Timestamp object whose date is in 2023', () => {
+    const timestamp = new Firestore.Timestamp(1680272000, 840000000);
+    expect(JSON.stringify(timestamp, Firestore.Timestamp.replacer)).to.equal('"2023-03-31T14:13:20.840000000Z"');
+  });
+
+  it.only('JSON.stringify() on the smallest Timestamp object', () => {
+    const timestamp = new Firestore.Timestamp(-62135596800, 0);
+    const writeResult = new Firestore.WriteResult(timestamp);
+    expect(JSON.stringify(writeResult, Firestore.WriteResult.replacer)).to.equal('{"writeTime":"\\"0001-01-01T00:00:00.000000000Z\\""}');
   });
 });

types/firestore.d.ts

@@ -1206,6 +1206,9 @@ declare namespace FirebaseFirestore {
   export class WriteResult {
     private constructor();
 
+    static reviver(this: any, key: string, value: any): any;
+    static replacer(this: any, key: string, value: any): any;
+
     /**
      * The write time as set by the Firestore servers.
      */