Preserve nano resolution when converting to ISO string.

Author: yuchenshi

packages/firestore/src/api/timestamp.ts

@@ -83,6 +83,14 @@ export class Timestamp {
     );
   }
 
+  _toISOString(): string {
+    const strMillis = this.toDate().toISOString(); // JS Date only has millis.
+    // So let's replace the fraction w/ up to 9 digits (nano resolution).
+    const fracs = (this.nanoseconds / 1e9).toFixed(9).substr(1); // .123456789
+    const strNanos = strMillis.replace(/\.\d*/, '').replace('Z', fracs + 'Z');
+    return strNanos;
+  }
+
   toString(): string {
     return (
       'Timestamp(seconds=' +

packages/firestore/src/remote/serializer.ts

@@ -208,7 +208,7 @@ export class JsonProtoSerializer {
    */
   private toTimestamp(timestamp: Timestamp): string {
     if (this.options.useProto3Json) {
-      return timestamp.toDate().toISOString();
+      return timestamp._toISOString();
     } else {
       return {
         seconds: '' + timestamp.seconds,

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

@@ -33,4 +33,13 @@ describe('Timestamp', () => {
       nanoseconds: 750000000
     });
   });
+
+  it('_toISOString', () => {
+    expect(new Timestamp(0, 1001)._toISOString()).to.equal(
+      '1970-01-01T00:00:00.000001001Z');
+    expect(new Timestamp(0, 1000)._toISOString()).to.equal(
+      '1970-01-01T00:00:00.000001000Z');
+    expect(new Timestamp(1570664492, 537000000)._toISOString()).to.equal(
+      '2019-10-09T23:41:32.537000000Z');
+  });
 });