fix JTD serializer for Date objects, fixes #1691

Author: epoberezkin

lib/compile/jtd/serialize.ts

@@ -213,7 +213,7 @@ function serializeType(cxt: SerializeCxt): void {
     case "timestamp":
       gen.if(
         _`${data} instanceof Date`,
-        () => gen.add(N.json, _`${data}.toISOString()`),
+        () => gen.add(N.json, _`'"' + ${data}.toISOString() + '"'`),
         () => serializeString(cxt)
       )
       break

spec/jtd-timestamps.spec.ts

@@ -67,4 +67,16 @@ describe("JTD timestamps", function () {
       assert.strictEqual(parseTS('"2021-05-14"')?.toISOString(), "2021-05-14T00:00:00.000Z")
     })
   })
+
+  describe("serializing Date objects", () => {
+    it("should serialize Date as JSON string", () => {
+      const schema: JTDSchemaType<Date> = {type: "timestamp"}
+      const ajv = new _AjvJTD()
+      const serializeTS = ajv.compileSerializer(schema)
+      assert.strictEqual(
+        serializeTS(new Date("2021-05-14T17:59:03.851Z")),
+        '"2021-05-14T17:59:03.851Z"'
+      )
+    })
+  })
 })