Add toJSON methods to GeoPoint and Timestamp (#3615)

`GeoPoint` and `Timestamp` are the only classes in the public API that have a well-defined JSON representation.

Fixes #3605

Author: var-const

packages/firestore/src/api/geo_point.ts

@@ -71,6 +71,10 @@ export class GeoPoint {
     return this._lat === other._lat && this._long === other._long;
   }
 
+  toJSON(): { latitude: number; longitude: number } {
+    return { latitude: this._lat, longitude: this._long };
+  }
+
   /**
    * Actually private to JS consumers of our API, so this function is prefixed
    * with an underscore.

packages/firestore/src/api/timestamp.ts

@@ -95,6 +95,10 @@ export class Timestamp {
     );
   }
 
+  toJSON(): { seconds: number; nanoseconds: number } {
+    return { seconds: this.seconds, nanoseconds: this.nanoseconds };
+  }
+
   valueOf(): string {
     // This method returns a string of the form <seconds>.<nanoseconds> where <seconds> is
     // translated to have a non-negative value and both <seconds> and <nanoseconds> are left-padded

packages/firestore/test/unit/api/geo_point.test.ts

@@ -100,4 +100,19 @@ describe('GeoPoint', () => {
     expectNotEqual(new GeoPoint(1, 2), new GeoPoint(1, 1));
     expectNotEqual(new GeoPoint(1, 2), new GeoPoint(2, 1));
   });
+
+  it('serializes to JSON', () => {
+    expect(new GeoPoint(1, 2).toJSON()).to.deep.equal({
+      latitude: 1,
+      longitude: 2
+    });
+    expect(new GeoPoint(0, 0).toJSON()).to.deep.equal({
+      latitude: 0,
+      longitude: 0
+    });
+    expect(new GeoPoint(90, 180).toJSON()).to.deep.equal({
+      latitude: 90,
+      longitude: 180
+    });
+  });
 });

packages/firestore/test/unit/api/timestamp.test.ts

@@ -132,4 +132,19 @@ describe('Timestamp', () => {
     expect(t1 > t2).to.be.false;
     expect(t1 >= t2).to.be.false;
   });
+
+  it('serializes to JSON', () => {
+    expect(new Timestamp(123, 456).toJSON()).to.deep.equal({
+      seconds: 123,
+      nanoseconds: 456
+    });
+    expect(new Timestamp(0, 0).toJSON()).to.deep.equal({
+      seconds: 0,
+      nanoseconds: 0
+    });
+    expect(new Timestamp(-123, 456).toJSON()).to.deep.equal({
+      seconds: -123,
+      nanoseconds: 456
+    });
+  });
 });