diff --git a/CHANGELOG.md b/CHANGELOG.md
index ba2c3f2..aba159d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,10 @@
 
 - Fix JSON serializtion for Timestamp and Duration well known types. See [#39](https://github.com/tatethurston/ProtoScript/issues/39).
 - Accept all value permutations as described by the Proto3 JSON spec when parsing JSON messages.
+- #initialize now accepts partial messages. This enables you to create a full message with select fields set to a user-provided value:
+  ```ts
+  const ron = User.initialize({ firstName: "Ron" });
+  ```
 
 ## v0.0.18
 
diff --git a/README.md b/README.md
index 4d12a37..27502ed 100644
--- a/README.md
+++ b/README.md
@@ -76,44 +76,40 @@ Using the `User` generated by ProtoScript will look like this:
 ```typescript
 import { User, UserJSON } from "./user.pb.js";
 
-// protocol buffers
-const bytes = User.encode({
-  firstName: "Homer",
-  lastName: "Simpson",
+const user = {
+  firstName: "Harry",
+  lastName: "Potter",
   active: true,
-  locations: ["Springfield"],
-  projects: { SPP: "Springfield Power Plant" },
+  locations: ["Hogwarts"],
+  projects: { DA: "Dumbledore's Army },
   manager: {
-    firstName: "Montgomery",
-    lastName: "Burns",
-  },
-});
-console.log(bytes);
+    firstName: "Albus",
+    lastName: "Dumbledore",
+  }
+}
+
+// protocol buffers
+const bytes = User.encode(user);
+console.log(bytes); // Uint8Array
 
 const userFromBytes = User.decode(bytes);
-console.log(userFromBytes);
+console.log(userFromBytes); // our user above
 
 // json
-const json = UserJSON.encode({
-  firstName: "Homer",
-  lastName: "Simpson",
-  active: true,
-  locations: ["Springfield"],
-  projects: { SPP: "Springfield Power Plant" },
-  manager: {
-    firstName: "Montgomery",
-    lastName: "Burns",
-  },
-});
-console.log(json);
+const json = UserJSON.encode(user)
+console.log(json); // json string
 
 const userFromJSON = UserJSON.decode(json);
-console.log(userFromJSON);
+console.log(userFromJSON); // our user above
 
 // ProtoScript generates and consumes plain JavaScript objects (POJOs) over classes. If you want to generate a full message
 // with default fields, you can use the #initialize method on the generated message class:
 const user = User.initialize();
 console.log(user);
+
+// the generated object is just a POJO. You can encode any POJO as protobuf or json as long as it conforms to a subset of the message interface defined in your .proto:
+console.log(User.encode(user))
+console.log(UserJSON.encode(user))
 ```
 
 ## Installation 📦
diff --git a/e2e/conformance/proto/conformance/conformance.pb.ts b/e2e/conformance/proto/conformance/conformance.pb.ts
index 847ca28..659a95a 100644
--- a/e2e/conformance/proto/conformance/conformance.pb.ts
+++ b/e2e/conformance/proto/conformance/conformance.pb.ts
@@ -313,9 +313,10 @@ export const FailureSet = {
   /**
    * Initializes FailureSet with all fields set to their default value.
    */
-  initialize: function (): FailureSet {
+  initialize: function (msg?: Partial<FailureSet>): FailureSet {
     return {
       failure: [],
+      ...msg,
     };
   },
 
@@ -380,7 +381,7 @@ export const ConformanceRequest = {
   /**
    * Initializes ConformanceRequest with all fields set to their default value.
    */
-  initialize: function (): ConformanceRequest {
+  initialize: function (msg?: Partial<ConformanceRequest>): ConformanceRequest {
     return {
       protobufPayload: undefined,
       jsonPayload: undefined,
@@ -391,6 +392,7 @@ export const ConformanceRequest = {
       testCategory: TestCategory._fromInt(0),
       jspbEncodingOptions: JspbEncodingConfig.initialize(),
       printUnknownFields: false,
+      ...msg,
     };
   },
 
@@ -521,7 +523,9 @@ export const ConformanceResponse = {
   /**
    * Initializes ConformanceResponse with all fields set to their default value.
    */
-  initialize: function (): ConformanceResponse {
+  initialize: function (
+    msg?: Partial<ConformanceResponse>,
+  ): ConformanceResponse {
     return {
       parseError: undefined,
       serializeError: undefined,
@@ -532,6 +536,7 @@ export const ConformanceResponse = {
       skipped: undefined,
       jspbPayload: undefined,
       textPayload: undefined,
+      ...msg,
     };
   },
 
@@ -652,9 +657,10 @@ export const JspbEncodingConfig = {
   /**
    * Initializes JspbEncodingConfig with all fields set to their default value.
    */
-  initialize: function (): JspbEncodingConfig {
+  initialize: function (msg?: Partial<JspbEncodingConfig>): JspbEncodingConfig {
     return {
       useJspbArrayAnyFormat: false,
+      ...msg,
     };
   },
 
@@ -862,9 +868,10 @@ export const FailureSetJSON = {
   /**
    * Initializes FailureSet with all fields set to their default value.
    */
-  initialize: function (): FailureSet {
+  initialize: function (msg?: Partial<FailureSet>): FailureSet {
     return {
       failure: [],
+      ...msg,
     };
   },
 
@@ -914,7 +921,7 @@ export const ConformanceRequestJSON = {
   /**
    * Initializes ConformanceRequest with all fields set to their default value.
    */
-  initialize: function (): ConformanceRequest {
+  initialize: function (msg?: Partial<ConformanceRequest>): ConformanceRequest {
     return {
       protobufPayload: undefined,
       jsonPayload: undefined,
@@ -925,6 +932,7 @@ export const ConformanceRequestJSON = {
       testCategory: TestCategory._fromInt(0),
       jspbEncodingOptions: JspbEncodingConfigJSON.initialize(),
       printUnknownFields: false,
+      ...msg,
     };
   },
 
@@ -1048,7 +1056,9 @@ export const ConformanceResponseJSON = {
   /**
    * Initializes ConformanceResponse with all fields set to their default value.
    */
-  initialize: function (): ConformanceResponse {
+  initialize: function (
+    msg?: Partial<ConformanceResponse>,
+  ): ConformanceResponse {
     return {
       parseError: undefined,
       serializeError: undefined,
@@ -1059,6 +1069,7 @@ export const ConformanceResponseJSON = {
       skipped: undefined,
       jspbPayload: undefined,
       textPayload: undefined,
+      ...msg,
     };
   },
 
@@ -1168,9 +1179,10 @@ export const JspbEncodingConfigJSON = {
   /**
    * Initializes JspbEncodingConfig with all fields set to their default value.
    */
-  initialize: function (): JspbEncodingConfig {
+  initialize: function (msg?: Partial<JspbEncodingConfig>): JspbEncodingConfig {
     return {
       useJspbArrayAnyFormat: false,
+      ...msg,
     };
   },
 
diff --git a/e2e/conformance/proto/google/protobuf/test_messages_proto3.pb.ts b/e2e/conformance/proto/google/protobuf/test_messages_proto3.pb.ts
index 9537905..7357388 100644
--- a/e2e/conformance/proto/google/protobuf/test_messages_proto3.pb.ts
+++ b/e2e/conformance/proto/google/protobuf/test_messages_proto3.pb.ts
@@ -451,7 +451,7 @@ export const TestAllTypesProto3 = {
   /**
    * Initializes TestAllTypesProto3 with all fields set to their default value.
    */
-  initialize: function (): TestAllTypesProto3 {
+  initialize: function (msg?: Partial<TestAllTypesProto3>): TestAllTypesProto3 {
     return {
       optionalInt32: 0,
       optionalInt64: 0n,
@@ -604,6 +604,7 @@ export const TestAllTypesProto3 = {
       fieldName16: 0,
       fieldName17: 0,
       FieldName18: 0,
+      ...msg,
     };
   },
 
@@ -2602,10 +2603,13 @@ export const TestAllTypesProto3 = {
     /**
      * Initializes TestAllTypesProto3.NestedMessage with all fields set to their default value.
      */
-    initialize: function (): TestAllTypesProto3.NestedMessage {
+    initialize: function (
+      msg?: Partial<TestAllTypesProto3.NestedMessage>,
+    ): TestAllTypesProto3.NestedMessage {
       return {
         a: 0,
         corecursive: undefined,
+        ...msg,
       };
     },
 
@@ -3552,9 +3556,10 @@ export const ForeignMessage = {
   /**
    * Initializes ForeignMessage with all fields set to their default value.
    */
-  initialize: function (): ForeignMessage {
+  initialize: function (msg?: Partial<ForeignMessage>): ForeignMessage {
     return {
       c: 0,
+      ...msg,
     };
   },
 
@@ -3613,8 +3618,12 @@ export const NullHypothesisProto3 = {
   /**
    * Initializes NullHypothesisProto3 with all fields set to their default value.
    */
-  initialize: function (): NullHypothesisProto3 {
-    return {};
+  initialize: function (
+    msg?: Partial<NullHypothesisProto3>,
+  ): NullHypothesisProto3 {
+    return {
+      ...msg,
+    };
   },
 
   /**
@@ -3656,8 +3665,10 @@ export const EnumOnlyProto3 = {
   /**
    * Initializes EnumOnlyProto3 with all fields set to their default value.
    */
-  initialize: function (): EnumOnlyProto3 {
-    return {};
+  initialize: function (msg?: Partial<EnumOnlyProto3>): EnumOnlyProto3 {
+    return {
+      ...msg,
+    };
   },
 
   /**
@@ -3791,7 +3802,7 @@ export const TestAllTypesProto3JSON = {
   /**
    * Initializes TestAllTypesProto3 with all fields set to their default value.
    */
-  initialize: function (): TestAllTypesProto3 {
+  initialize: function (msg?: Partial<TestAllTypesProto3>): TestAllTypesProto3 {
     return {
       optionalInt32: 0,
       optionalInt64: 0n,
@@ -3944,6 +3955,7 @@ export const TestAllTypesProto3JSON = {
       fieldName16: 0,
       fieldName17: 0,
       FieldName18: 0,
+      ...msg,
     };
   },
 
@@ -5739,10 +5751,13 @@ export const TestAllTypesProto3JSON = {
     /**
      * Initializes TestAllTypesProto3.NestedMessage with all fields set to their default value.
      */
-    initialize: function (): TestAllTypesProto3.NestedMessage {
+    initialize: function (
+      msg?: Partial<TestAllTypesProto3.NestedMessage>,
+    ): TestAllTypesProto3.NestedMessage {
       return {
         a: 0,
         corecursive: undefined,
+        ...msg,
       };
     },
 
@@ -6501,9 +6516,10 @@ export const ForeignMessageJSON = {
   /**
    * Initializes ForeignMessage with all fields set to their default value.
    */
-  initialize: function (): ForeignMessage {
+  initialize: function (msg?: Partial<ForeignMessage>): ForeignMessage {
     return {
       c: 0,
+      ...msg,
     };
   },
 
@@ -6550,8 +6566,12 @@ export const NullHypothesisProto3JSON = {
   /**
    * Initializes NullHypothesisProto3 with all fields set to their default value.
    */
-  initialize: function (): NullHypothesisProto3 {
-    return {};
+  initialize: function (
+    msg?: Partial<NullHypothesisProto3>,
+  ): NullHypothesisProto3 {
+    return {
+      ...msg,
+    };
   },
 
   /**
@@ -6592,8 +6612,10 @@ export const EnumOnlyProto3JSON = {
   /**
    * Initializes EnumOnlyProto3 with all fields set to their default value.
    */
-  initialize: function (): EnumOnlyProto3 {
-    return {};
+  initialize: function (msg?: Partial<EnumOnlyProto3>): EnumOnlyProto3 {
+    return {
+      ...msg,
+    };
   },
 
   /**
diff --git a/e2e/serialization/json.pb.ts b/e2e/serialization/json.pb.ts
index 2df9d27..5b1d1da 100644
--- a/e2e/serialization/json.pb.ts
+++ b/e2e/serialization/json.pb.ts
@@ -147,10 +147,11 @@ export const Message = {
   /**
    * Initializes Message with all fields set to their default value.
    */
-  initialize: function (): Message {
+  initialize: function (msg?: Partial<Message>): Message {
     return {
       fooBar: "",
       g: 0n,
+      ...msg,
     };
   },
 
@@ -222,7 +223,7 @@ export const SampleMessage = {
   /**
    * Initializes SampleMessage with all fields set to their default value.
    */
-  initialize: function (): SampleMessage {
+  initialize: function (msg?: Partial<SampleMessage>): SampleMessage {
     return {
       sampleMessage: Message.initialize(),
       sampleEnum: Enum._fromInt(0),
@@ -242,6 +243,7 @@ export const SampleMessage = {
       sampleTimestamp: protoscript.Timestamp.initialize(),
       sampleDuration: protoscript.Duration.initialize(),
       sampleStruct: protoscript.Struct.initialize(),
+      ...msg,
     };
   },
 
@@ -497,10 +499,11 @@ export const OptionalMessage = {
   /**
    * Initializes OptionalMessage with all fields set to their default value.
    */
-  initialize: function (): OptionalMessage {
+  initialize: function (msg?: Partial<OptionalMessage>): OptionalMessage {
     return {
       fooBar: undefined,
       g: undefined,
+      ...msg,
     };
   },
 
@@ -572,7 +575,9 @@ export const OptionalSampleMessage = {
   /**
    * Initializes OptionalSampleMessage with all fields set to their default value.
    */
-  initialize: function (): OptionalSampleMessage {
+  initialize: function (
+    msg?: Partial<OptionalSampleMessage>,
+  ): OptionalSampleMessage {
     return {
       sampleMessage: undefined,
       sampleEnum: undefined,
@@ -592,6 +597,7 @@ export const OptionalSampleMessage = {
       sampleTimestamp: undefined,
       sampleDuration: undefined,
       sampleStruct: undefined,
+      ...msg,
     };
   },
 
@@ -888,10 +894,11 @@ export const MessageJSON = {
   /**
    * Initializes Message with all fields set to their default value.
    */
-  initialize: function (): Message {
+  initialize: function (msg?: Partial<Message>): Message {
     return {
       fooBar: "",
       g: 0n,
+      ...msg,
     };
   },
 
@@ -946,7 +953,7 @@ export const SampleMessageJSON = {
   /**
    * Initializes SampleMessage with all fields set to their default value.
    */
-  initialize: function (): SampleMessage {
+  initialize: function (msg?: Partial<SampleMessage>): SampleMessage {
     return {
       sampleMessage: MessageJSON.initialize(),
       sampleEnum: Enum._fromInt(0),
@@ -966,6 +973,7 @@ export const SampleMessageJSON = {
       sampleTimestamp: protoscript.TimestampJSON.initialize(),
       sampleDuration: protoscript.DurationJSON.initialize(),
       sampleStruct: protoscript.StructJSON.initialize(),
+      ...msg,
     };
   },
 
@@ -1204,10 +1212,11 @@ export const OptionalMessageJSON = {
   /**
    * Initializes OptionalMessage with all fields set to their default value.
    */
-  initialize: function (): OptionalMessage {
+  initialize: function (msg?: Partial<OptionalMessage>): OptionalMessage {
     return {
       fooBar: undefined,
       g: undefined,
+      ...msg,
     };
   },
 
@@ -1264,7 +1273,9 @@ export const OptionalSampleMessageJSON = {
   /**
    * Initializes OptionalSampleMessage with all fields set to their default value.
    */
-  initialize: function (): OptionalSampleMessage {
+  initialize: function (
+    msg?: Partial<OptionalSampleMessage>,
+  ): OptionalSampleMessage {
     return {
       sampleMessage: undefined,
       sampleEnum: undefined,
@@ -1284,6 +1295,7 @@ export const OptionalSampleMessageJSON = {
       sampleTimestamp: undefined,
       sampleDuration: undefined,
       sampleStruct: undefined,
+      ...msg,
     };
   },
 
diff --git a/e2e/serialization/message.pb.ts b/e2e/serialization/message.pb.ts
index 8bbebe7..6092cdf 100644
--- a/e2e/serialization/message.pb.ts
+++ b/e2e/serialization/message.pb.ts
@@ -129,7 +129,7 @@ export const Foo = {
   /**
    * Initializes Foo with all fields set to their default value.
    */
-  initialize: function (): Foo {
+  initialize: function (msg?: Partial<Foo>): Foo {
     return {
       fieldOne: undefined,
       fieldTwo: {},
@@ -146,6 +146,7 @@ export const Foo = {
       fieldThirteen: undefined,
       fieldFourteen: undefined,
       fieldFifteen: [],
+      ...msg,
     };
   },
 
@@ -336,11 +337,12 @@ export const Foo = {
     /**
      * Initializes Foo.FooBar with all fields set to their default value.
      */
-    initialize: function (): Foo.FooBar {
+    initialize: function (msg?: Partial<Foo.FooBar>): Foo.FooBar {
       return {
         fieldOne: "",
         fieldTwo: {},
         fieldThree: [],
+        ...msg,
       };
     },
 
@@ -524,11 +526,12 @@ export const Bar = {
   /**
    * Initializes Bar with all fields set to their default value.
    */
-  initialize: function (): Bar {
+  initialize: function (msg?: Partial<Bar>): Bar {
     return {
       fieldOne: "",
       fieldTwo: {},
       fieldThree: [],
+      ...msg,
     };
   },
 
@@ -699,7 +702,7 @@ export const FooJSON = {
   /**
    * Initializes Foo with all fields set to their default value.
    */
-  initialize: function (): Foo {
+  initialize: function (msg?: Partial<Foo>): Foo {
     return {
       fieldOne: undefined,
       fieldTwo: {},
@@ -716,6 +719,7 @@ export const FooJSON = {
       fieldThirteen: undefined,
       fieldFourteen: undefined,
       fieldFifteen: [],
+      ...msg,
     };
   },
 
@@ -895,11 +899,12 @@ export const FooJSON = {
     /**
      * Initializes Foo.FooBar with all fields set to their default value.
      */
-    initialize: function (): Foo.FooBar {
+    initialize: function (msg?: Partial<Foo.FooBar>): Foo.FooBar {
       return {
         fieldOne: "",
         fieldTwo: {},
         fieldThree: [],
+        ...msg,
       };
     },
 
@@ -1046,11 +1051,12 @@ export const BarJSON = {
   /**
    * Initializes Bar with all fields set to their default value.
    */
-  initialize: function (): Bar {
+  initialize: function (msg?: Partial<Bar>): Bar {
     return {
       fieldOne: "",
       fieldTwo: {},
       fieldThree: [],
+      ...msg,
     };
   },
 
diff --git a/e2e/treeshaking/treeshaking.pb.ts b/e2e/treeshaking/treeshaking.pb.ts
index b10bae1..a05c192 100644
--- a/e2e/treeshaking/treeshaking.pb.ts
+++ b/e2e/treeshaking/treeshaking.pb.ts
@@ -58,7 +58,7 @@ export const TreeshakingTest = {
   /**
    * Initializes TreeshakingTest with all fields set to their default value.
    */
-  initialize: function (): TreeshakingTest {
+  initialize: function (msg?: Partial<TreeshakingTest>): TreeshakingTest {
     return {
       stringField: "",
       repeatedStringField: [],
@@ -67,6 +67,7 @@ export const TreeshakingTest = {
       optionalMessageField: undefined,
       timestampField: protoscript.Timestamp.initialize(),
       mapField: {},
+      ...msg,
     };
   },
 
@@ -248,9 +249,10 @@ export const NestedMessage = {
   /**
    * Initializes NestedMessage with all fields set to their default value.
    */
-  initialize: function (): NestedMessage {
+  initialize: function (msg?: Partial<NestedMessage>): NestedMessage {
     return {
       stringField: undefined,
+      ...msg,
     };
   },
 
@@ -316,7 +318,7 @@ export const TreeshakingTestJSON = {
   /**
    * Initializes TreeshakingTest with all fields set to their default value.
    */
-  initialize: function (): TreeshakingTest {
+  initialize: function (msg?: Partial<TreeshakingTest>): TreeshakingTest {
     return {
       stringField: "",
       repeatedStringField: [],
@@ -325,6 +327,7 @@ export const TreeshakingTestJSON = {
       optionalMessageField: undefined,
       timestampField: protoscript.TimestampJSON.initialize(),
       mapField: {},
+      ...msg,
     };
   },
 
@@ -487,9 +490,10 @@ export const NestedMessageJSON = {
   /**
    * Initializes NestedMessage with all fields set to their default value.
    */
-  initialize: function (): NestedMessage {
+  initialize: function (msg?: Partial<NestedMessage>): NestedMessage {
     return {
       stringField: undefined,
+      ...msg,
     };
   },
 
diff --git a/examples/closure-compiler/src/haberdasher.pb.ts b/examples/closure-compiler/src/haberdasher.pb.ts
index 4512ec5..5f4ddd6 100644
--- a/examples/closure-compiler/src/haberdasher.pb.ts
+++ b/examples/closure-compiler/src/haberdasher.pb.ts
@@ -62,9 +62,10 @@ export const Size = {
   /**
    * Initializes Size with all fields set to their default value.
    */
-  initialize: function (): Size {
+  initialize: function (msg?: Partial<Size>): Size {
     return {
       inches: 0,
+      ...msg,
     };
   },
 
@@ -126,11 +127,12 @@ export const Hat = {
   /**
    * Initializes Hat with all fields set to their default value.
    */
-  initialize: function (): Hat {
+  initialize: function (msg?: Partial<Hat>): Hat {
     return {
       inches: 0,
       color: "",
       name: "",
+      ...msg,
     };
   },
 
@@ -204,9 +206,10 @@ export const SizeJSON = {
   /**
    * Initializes Size with all fields set to their default value.
    */
-  initialize: function (): Size {
+  initialize: function (msg?: Partial<Size>): Size {
     return {
       inches: 0,
+      ...msg,
     };
   },
 
@@ -251,11 +254,12 @@ export const HatJSON = {
   /**
    * Initializes Hat with all fields set to their default value.
    */
-  initialize: function (): Hat {
+  initialize: function (msg?: Partial<Hat>): Hat {
     return {
       inches: 0,
       color: "",
       name: "",
+      ...msg,
     };
   },
 
diff --git a/examples/javascript/src/haberdasher.pb.js b/examples/javascript/src/haberdasher.pb.js
index 9eecc67..dfce397 100644
--- a/examples/javascript/src/haberdasher.pb.js
+++ b/examples/javascript/src/haberdasher.pb.js
@@ -32,9 +32,10 @@ export const Size = {
   /**
    * Initializes Size with all fields set to their default value.
    */
-  initialize: function () {
+  initialize: function (msg) {
     return {
       inches: 0,
+      ...msg,
     };
   },
 
@@ -93,11 +94,12 @@ export const Hat = {
   /**
    * Initializes Hat with all fields set to their default value.
    */
-  initialize: function () {
+  initialize: function (msg) {
     return {
       inches: 0,
       color: "",
       name: "",
+      ...msg,
     };
   },
 
@@ -168,9 +170,10 @@ export const SizeJSON = {
   /**
    * Initializes Size with all fields set to their default value.
    */
-  initialize: function () {
+  initialize: function (msg) {
     return {
       inches: 0,
+      ...msg,
     };
   },
 
@@ -215,11 +218,12 @@ export const HatJSON = {
   /**
    * Initializes Hat with all fields set to their default value.
    */
-  initialize: function () {
+  initialize: function (msg) {
     return {
       inches: 0,
       color: "",
       name: "",
+      ...msg,
     };
   },
 
diff --git a/examples/protoc/src/haberdasher.pb.js b/examples/protoc/src/haberdasher.pb.js
index 9eecc67..dfce397 100644
--- a/examples/protoc/src/haberdasher.pb.js
+++ b/examples/protoc/src/haberdasher.pb.js
@@ -32,9 +32,10 @@ export const Size = {
   /**
    * Initializes Size with all fields set to their default value.
    */
-  initialize: function () {
+  initialize: function (msg) {
     return {
       inches: 0,
+      ...msg,
     };
   },
 
@@ -93,11 +94,12 @@ export const Hat = {
   /**
    * Initializes Hat with all fields set to their default value.
    */
-  initialize: function () {
+  initialize: function (msg) {
     return {
       inches: 0,
       color: "",
       name: "",
+      ...msg,
     };
   },
 
@@ -168,9 +170,10 @@ export const SizeJSON = {
   /**
    * Initializes Size with all fields set to their default value.
    */
-  initialize: function () {
+  initialize: function (msg) {
     return {
       inches: 0,
+      ...msg,
     };
   },
 
@@ -215,11 +218,12 @@ export const HatJSON = {
   /**
    * Initializes Hat with all fields set to their default value.
    */
-  initialize: function () {
+  initialize: function (msg) {
     return {
       inches: 0,
       color: "",
       name: "",
+      ...msg,
     };
   },
 
diff --git a/examples/typescript/src/haberdasher.pb.ts b/examples/typescript/src/haberdasher.pb.ts
index 0a8b61e..009c94f 100644
--- a/examples/typescript/src/haberdasher.pb.ts
+++ b/examples/typescript/src/haberdasher.pb.ts
@@ -54,11 +54,12 @@ export const Hat = {
   /**
    * Initializes Hat with all fields set to their default value.
    */
-  initialize: function (): Hat {
+  initialize: function (msg?: Partial<Hat>): Hat {
     return {
       size: srcSize.Size.initialize(),
       color: "",
       name: "",
+      ...msg,
     };
   },
 
@@ -132,11 +133,12 @@ export const HatJSON = {
   /**
    * Initializes Hat with all fields set to their default value.
    */
-  initialize: function (): Hat {
+  initialize: function (msg?: Partial<Hat>): Hat {
     return {
       size: srcSize.SizeJSON.initialize(),
       color: "",
       name: "",
+      ...msg,
     };
   },
 
diff --git a/examples/typescript/src/size.pb.ts b/examples/typescript/src/size.pb.ts
index 7c6b3fc..02412a3 100644
--- a/examples/typescript/src/size.pb.ts
+++ b/examples/typescript/src/size.pb.ts
@@ -47,9 +47,10 @@ export const Size = {
   /**
    * Initializes Size with all fields set to their default value.
    */
-  initialize: function (): Size {
+  initialize: function (msg?: Partial<Size>): Size {
     return {
       inches: 0,
+      ...msg,
     };
   },
 
@@ -109,9 +110,10 @@ export const SizeJSON = {
   /**
    * Initializes Size with all fields set to their default value.
    */
-  initialize: function (): Size {
+  initialize: function (msg?: Partial<Size>): Size {
     return {
       inches: 0,
+      ...msg,
     };
   },
 
diff --git a/examples/typescript/src/user.pb.ts b/examples/typescript/src/user.pb.ts
index 3a7d1fe..f036263 100644
--- a/examples/typescript/src/user.pb.ts
+++ b/examples/typescript/src/user.pb.ts
@@ -53,7 +53,7 @@ export const User = {
   /**
    * Initializes User with all fields set to their default value.
    */
-  initialize: function (): User {
+  initialize: function (msg?: Partial<User>): User {
     return {
       firstName: "",
       lastName: "",
@@ -61,6 +61,7 @@ export const User = {
       manager: undefined,
       locations: [],
       projects: {},
+      ...msg,
     };
   },
 
@@ -210,7 +211,7 @@ export const UserJSON = {
   /**
    * Initializes User with all fields set to their default value.
    */
-  initialize: function (): User {
+  initialize: function (msg?: Partial<User>): User {
     return {
       firstName: "",
       lastName: "",
@@ -218,6 +219,7 @@ export const UserJSON = {
       manager: undefined,
       locations: [],
       projects: {},
+      ...msg,
     };
   },
 
diff --git a/packages/protoscript/src/codegen/autogenerate/index.ts b/packages/protoscript/src/codegen/autogenerate/index.ts
index 26658fe..79fb340 100644
--- a/packages/protoscript/src/codegen/autogenerate/index.ts
+++ b/packages/protoscript/src/codegen/autogenerate/index.ts
@@ -149,9 +149,9 @@ function writeProtobufSerializers(
              node.content.namespacedName
            } with all fields set to their default value.
            */
-          initialize: function()${printIfTypescript(
-            `: ${node.content.namespacedName}`,
-          )} {
+          initialize: function(msg${printIfTypescript(
+            `?: Partial<${node.content.namespacedName}>`,
+          )}) ${printIfTypescript(`:${node.content.namespacedName}`)} {
             return {
               ${node.content.fields
                 .map((field) => {
@@ -175,7 +175,8 @@ function writeProtobufSerializers(
                     return `${field.name}: ${field.defaultValue},`;
                   }
                 })
-                .join("")}
+                .join("\n")}
+                ...msg,
             };`;
           result += "},\n\n";
         }
@@ -488,9 +489,9 @@ function writeJSONSerializers(types: ProtoTypes[], parents: string[]): string {
              node.content.namespacedName
            } with all fields set to their default value.
            */
-          initialize: function()${printIfTypescript(
-            `: ${node.content.namespacedName}`,
-          )} {
+          initialize: function(msg${printIfTypescript(
+            `?: Partial<${node.content.namespacedName}>`,
+          )}) ${printIfTypescript(`:${node.content.namespacedName}`)} {
             return {
               ${node.content.fields
                 .map((field) => {
@@ -514,7 +515,8 @@ function writeJSONSerializers(types: ProtoTypes[], parents: string[]): string {
                     return `${field.name}: ${field.defaultValue},`;
                   }
                 })
-                .join("")}
+                .join("\n")}
+                ...msg
             };`;
           result += "},\n\n";
         }
diff --git a/packages/protoscript/src/runtime/well-known-types/any.pb.ts b/packages/protoscript/src/runtime/well-known-types/any.pb.ts
index 99184e7..493e548 100644
--- a/packages/protoscript/src/runtime/well-known-types/any.pb.ts
+++ b/packages/protoscript/src/runtime/well-known-types/any.pb.ts
@@ -164,10 +164,11 @@ export const Any = {
   /**
    * Initializes Any with all fields set to their default value.
    */
-  initialize: function (): Any {
+  initialize: function (msg?: Partial<Any>): Any {
     return {
       typeUrl: "",
       value: new Uint8Array(),
+      ...msg,
     };
   },
 
@@ -234,10 +235,11 @@ export const AnyJSON = {
   /**
    * Initializes Any with all fields set to their default value.
    */
-  initialize: function (): Any {
+  initialize: function (msg?: Partial<Any>): Any {
     return {
       typeUrl: "",
       value: new Uint8Array(),
+      ...msg,
     };
   },
 
diff --git a/packages/protoscript/src/runtime/well-known-types/api.pb.ts b/packages/protoscript/src/runtime/well-known-types/api.pb.ts
index a1dec96..d7a20b7 100644
--- a/packages/protoscript/src/runtime/well-known-types/api.pb.ts
+++ b/packages/protoscript/src/runtime/well-known-types/api.pb.ts
@@ -226,7 +226,7 @@ export const Api = {
   /**
    * Initializes Api with all fields set to their default value.
    */
-  initialize: function (): Api {
+  initialize: function (msg?: Partial<Api>): Api {
     return {
       name: "",
       methods: [],
@@ -235,6 +235,7 @@ export const Api = {
       sourceContext: protoscript.SourceContext.initialize(),
       mixins: [],
       syntax: protoscript.Syntax._fromInt(0),
+      ...msg,
     };
   },
 
@@ -355,7 +356,7 @@ export const Method = {
   /**
    * Initializes Method with all fields set to their default value.
    */
-  initialize: function (): Method {
+  initialize: function (msg?: Partial<Method>): Method {
     return {
       name: "",
       requestTypeUrl: "",
@@ -364,6 +365,7 @@ export const Method = {
       responseStreaming: false,
       options: [],
       syntax: protoscript.Syntax._fromInt(0),
+      ...msg,
     };
   },
 
@@ -476,10 +478,11 @@ export const Mixin = {
   /**
    * Initializes Mixin with all fields set to their default value.
    */
-  initialize: function (): Mixin {
+  initialize: function (msg?: Partial<Mixin>): Mixin {
     return {
       name: "",
       root: "",
+      ...msg,
     };
   },
 
@@ -546,7 +549,7 @@ export const ApiJSON = {
   /**
    * Initializes Api with all fields set to their default value.
    */
-  initialize: function (): Api {
+  initialize: function (msg?: Partial<Api>): Api {
     return {
       name: "",
       methods: [],
@@ -555,6 +558,7 @@ export const ApiJSON = {
       sourceContext: protoscript.SourceContextJSON.initialize(),
       mixins: [],
       syntax: protoscript.Syntax._fromInt(0),
+      ...msg,
     };
   },
 
@@ -661,7 +665,7 @@ export const MethodJSON = {
   /**
    * Initializes Method with all fields set to their default value.
    */
-  initialize: function (): Method {
+  initialize: function (msg?: Partial<Method>): Method {
     return {
       name: "",
       requestTypeUrl: "",
@@ -670,6 +674,7 @@ export const MethodJSON = {
       responseStreaming: false,
       options: [],
       syntax: protoscript.Syntax._fromInt(0),
+      ...msg,
     };
   },
 
@@ -763,10 +768,11 @@ export const MixinJSON = {
   /**
    * Initializes Mixin with all fields set to their default value.
    */
-  initialize: function (): Mixin {
+  initialize: function (msg?: Partial<Mixin>): Mixin {
     return {
       name: "",
       root: "",
+      ...msg,
     };
   },
 
diff --git a/packages/protoscript/src/runtime/well-known-types/duration.pb.ts b/packages/protoscript/src/runtime/well-known-types/duration.pb.ts
index 1c861a0..936ad14 100644
--- a/packages/protoscript/src/runtime/well-known-types/duration.pb.ts
+++ b/packages/protoscript/src/runtime/well-known-types/duration.pb.ts
@@ -116,10 +116,11 @@ export const Duration = {
   /**
    * Initializes Duration with all fields set to their default value.
    */
-  initialize: function (): Duration {
+  initialize: function (msg?: Partial<Duration>): Duration {
     return {
       seconds: 0n,
       nanos: 0,
+      ...msg,
     };
   },
 
@@ -192,10 +193,11 @@ export const DurationJSON = {
   /**
    * Initializes Duration with all fields set to their default value.
    */
-  initialize: function (): Duration {
+  initialize: function (msg?: Partial<Duration>): Duration {
     return {
       seconds: 0n,
       nanos: 0,
+      ...msg,
     };
   },
 
diff --git a/packages/protoscript/src/runtime/well-known-types/empty.pb.ts b/packages/protoscript/src/runtime/well-known-types/empty.pb.ts
index 989040c..3c23265 100644
--- a/packages/protoscript/src/runtime/well-known-types/empty.pb.ts
+++ b/packages/protoscript/src/runtime/well-known-types/empty.pb.ts
@@ -43,8 +43,10 @@ export const Empty = {
   /**
    * Initializes Empty with all fields set to their default value.
    */
-  initialize: function (): Empty {
-    return {};
+  initialize: function (msg?: Partial<Empty>): Empty {
+    return {
+      ...msg,
+    };
   },
 
   /**
@@ -90,8 +92,10 @@ export const EmptyJSON = {
   /**
    * Initializes Empty with all fields set to their default value.
    */
-  initialize: function (): Empty {
-    return {};
+  initialize: function (msg?: Partial<Empty>): Empty {
+    return {
+      ...msg,
+    };
   },
 
   /**
diff --git a/packages/protoscript/src/runtime/well-known-types/field_mask.pb.ts b/packages/protoscript/src/runtime/well-known-types/field_mask.pb.ts
index 4abe5cf..96d4d97 100644
--- a/packages/protoscript/src/runtime/well-known-types/field_mask.pb.ts
+++ b/packages/protoscript/src/runtime/well-known-types/field_mask.pb.ts
@@ -245,9 +245,10 @@ export const FieldMask = {
   /**
    * Initializes FieldMask with all fields set to their default value.
    */
-  initialize: function (): FieldMask {
+  initialize: function (msg?: Partial<FieldMask>): FieldMask {
     return {
       paths: [],
+      ...msg,
     };
   },
 
@@ -313,9 +314,10 @@ export const FieldMaskJSON = {
   /**
    * Initializes FieldMask with all fields set to their default value.
    */
-  initialize: function (): FieldMask {
+  initialize: function (msg?: Partial<FieldMask>): FieldMask {
     return {
       paths: [],
+      ...msg,
     };
   },
 
diff --git a/packages/protoscript/src/runtime/well-known-types/source_context.pb.ts b/packages/protoscript/src/runtime/well-known-types/source_context.pb.ts
index a05dca1..11381fc 100644
--- a/packages/protoscript/src/runtime/well-known-types/source_context.pb.ts
+++ b/packages/protoscript/src/runtime/well-known-types/source_context.pb.ts
@@ -49,9 +49,10 @@ export const SourceContext = {
   /**
    * Initializes SourceContext with all fields set to their default value.
    */
-  initialize: function (): SourceContext {
+  initialize: function (msg?: Partial<SourceContext>): SourceContext {
     return {
       fileName: "",
+      ...msg,
     };
   },
 
@@ -117,9 +118,10 @@ export const SourceContextJSON = {
   /**
    * Initializes SourceContext with all fields set to their default value.
    */
-  initialize: function (): SourceContext {
+  initialize: function (msg?: Partial<SourceContext>): SourceContext {
     return {
       fileName: "",
+      ...msg,
     };
   },
 
diff --git a/packages/protoscript/src/runtime/well-known-types/struct.pb.ts b/packages/protoscript/src/runtime/well-known-types/struct.pb.ts
index d5386bf..addc398 100644
--- a/packages/protoscript/src/runtime/well-known-types/struct.pb.ts
+++ b/packages/protoscript/src/runtime/well-known-types/struct.pb.ts
@@ -151,9 +151,10 @@ export const Struct = {
   /**
    * Initializes Struct with all fields set to their default value.
    */
-  initialize: function (): Struct {
+  initialize: function (msg?: Partial<Struct>): Struct {
     return {
       fields: {},
+      ...msg,
     };
   },
 
@@ -273,7 +274,7 @@ export const Value = {
   /**
    * Initializes Value with all fields set to their default value.
    */
-  initialize: function (): Value {
+  initialize: function (msg?: Partial<Value>): Value {
     return {
       nullValue: undefined,
       numberValue: undefined,
@@ -281,6 +282,7 @@ export const Value = {
       boolValue: undefined,
       structValue: undefined,
       listValue: undefined,
+      ...msg,
     };
   },
 
@@ -379,9 +381,10 @@ export const ListValue = {
   /**
    * Initializes ListValue with all fields set to their default value.
    */
-  initialize: function (): ListValue {
+  initialize: function (msg?: Partial<ListValue>): ListValue {
     return {
       values: [],
+      ...msg,
     };
   },
 
@@ -481,9 +484,10 @@ export const StructJSON = {
   /**
    * Initializes Struct with all fields set to their default value.
    */
-  initialize: function (): Struct {
+  initialize: function (msg?: Partial<Struct>): Struct {
     return {
       fields: {},
+      ...msg,
     };
   },
 
@@ -577,7 +581,7 @@ export const ValueJSON = {
   /**
    * Initializes Value with all fields set to their default value.
    */
-  initialize: function (): Value {
+  initialize: function (msg?: Partial<Value>): Value {
     return {
       nullValue: undefined,
       numberValue: undefined,
@@ -585,6 +589,7 @@ export const ValueJSON = {
       boolValue: undefined,
       structValue: undefined,
       listValue: undefined,
+      ...msg,
     };
   },
 
@@ -671,9 +676,10 @@ export const ListValueJSON = {
   /**
    * Initializes ListValue with all fields set to their default value.
    */
-  initialize: function (): ListValue {
+  initialize: function (msg?: Partial<ListValue>): ListValue {
     return {
       values: [],
+      ...msg,
     };
   },
 
diff --git a/packages/protoscript/src/runtime/well-known-types/timestamp.pb.ts b/packages/protoscript/src/runtime/well-known-types/timestamp.pb.ts
index 6b91692..52eaa48 100644
--- a/packages/protoscript/src/runtime/well-known-types/timestamp.pb.ts
+++ b/packages/protoscript/src/runtime/well-known-types/timestamp.pb.ts
@@ -145,10 +145,11 @@ export const Timestamp = {
   /**
    * Initializes Timestamp with all fields set to their default value.
    */
-  initialize: function (): Timestamp {
+  initialize: function (msg?: Partial<Timestamp>): Timestamp {
     return {
       seconds: 0n,
       nanos: 0,
+      ...msg,
     };
   },
 
@@ -221,10 +222,11 @@ export const TimestampJSON = {
   /**
    * Initializes Timestamp with all fields set to their default value.
    */
-  initialize: function (): Timestamp {
+  initialize: function (msg?: Partial<Timestamp>): Timestamp {
     return {
       seconds: 0n,
       nanos: 0,
+      ...msg,
     };
   },
 
diff --git a/packages/protoscript/src/runtime/well-known-types/type.pb.ts b/packages/protoscript/src/runtime/well-known-types/type.pb.ts
index 61cb7cb..79d9b1c 100644
--- a/packages/protoscript/src/runtime/well-known-types/type.pb.ts
+++ b/packages/protoscript/src/runtime/well-known-types/type.pb.ts
@@ -283,7 +283,7 @@ export const Type = {
   /**
    * Initializes Type with all fields set to their default value.
    */
-  initialize: function (): Type {
+  initialize: function (msg?: Partial<Type>): Type {
     return {
       name: "",
       fields: [],
@@ -292,6 +292,7 @@ export const Type = {
       sourceContext: protoscript.SourceContext.initialize(),
       syntax: Syntax._fromInt(0),
       edition: "",
+      ...msg,
     };
   },
 
@@ -406,7 +407,7 @@ export const Field = {
   /**
    * Initializes Field with all fields set to their default value.
    */
-  initialize: function (): Field {
+  initialize: function (msg?: Partial<Field>): Field {
     return {
       kind: Field.Kind._fromInt(0),
       cardinality: Field.Cardinality._fromInt(0),
@@ -418,6 +419,7 @@ export const Field = {
       options: [],
       jsonName: "",
       defaultValue: "",
+      ...msg,
     };
   },
 
@@ -824,7 +826,7 @@ export const Enum = {
   /**
    * Initializes Enum with all fields set to their default value.
    */
-  initialize: function (): Enum {
+  initialize: function (msg?: Partial<Enum>): Enum {
     return {
       name: "",
       enumvalue: [],
@@ -832,6 +834,7 @@ export const Enum = {
       sourceContext: protoscript.SourceContext.initialize(),
       syntax: Syntax._fromInt(0),
       edition: "",
+      ...msg,
     };
   },
 
@@ -943,11 +946,12 @@ export const EnumValue = {
   /**
    * Initializes EnumValue with all fields set to their default value.
    */
-  initialize: function (): EnumValue {
+  initialize: function (msg?: Partial<EnumValue>): EnumValue {
     return {
       name: "",
       number: 0,
       options: [],
+      ...msg,
     };
   },
 
@@ -1028,10 +1032,11 @@ export const Option = {
   /**
    * Initializes Option with all fields set to their default value.
    */
-  initialize: function (): Option {
+  initialize: function (msg?: Partial<Option>): Option {
     return {
       name: "",
       value: protoscript.Any.initialize(),
+      ...msg,
     };
   },
 
@@ -1156,7 +1161,7 @@ export const TypeJSON = {
   /**
    * Initializes Type with all fields set to their default value.
    */
-  initialize: function (): Type {
+  initialize: function (msg?: Partial<Type>): Type {
     return {
       name: "",
       fields: [],
@@ -1165,6 +1170,7 @@ export const TypeJSON = {
       sourceContext: protoscript.SourceContextJSON.initialize(),
       syntax: Syntax._fromInt(0),
       edition: "",
+      ...msg,
     };
   },
 
@@ -1267,7 +1273,7 @@ export const FieldJSON = {
   /**
    * Initializes Field with all fields set to their default value.
    */
-  initialize: function (): Field {
+  initialize: function (msg?: Partial<Field>): Field {
     return {
       kind: Field.Kind._fromInt(0),
       cardinality: Field.Cardinality._fromInt(0),
@@ -1279,6 +1285,7 @@ export const FieldJSON = {
       options: [],
       jsonName: "",
       defaultValue: "",
+      ...msg,
     };
   },
 
@@ -1670,7 +1677,7 @@ export const EnumJSON = {
   /**
    * Initializes Enum with all fields set to their default value.
    */
-  initialize: function (): Enum {
+  initialize: function (msg?: Partial<Enum>): Enum {
     return {
       name: "",
       enumvalue: [],
@@ -1678,6 +1685,7 @@ export const EnumJSON = {
       sourceContext: protoscript.SourceContextJSON.initialize(),
       syntax: Syntax._fromInt(0),
       edition: "",
+      ...msg,
     };
   },
 
@@ -1776,11 +1784,12 @@ export const EnumValueJSON = {
   /**
    * Initializes EnumValue with all fields set to their default value.
    */
-  initialize: function (): EnumValue {
+  initialize: function (msg?: Partial<EnumValue>): EnumValue {
     return {
       name: "",
       number: 0,
       options: [],
+      ...msg,
     };
   },
 
@@ -1845,10 +1854,11 @@ export const OptionJSON = {
   /**
    * Initializes Option with all fields set to their default value.
    */
-  initialize: function (): Option {
+  initialize: function (msg?: Partial<Option>): Option {
     return {
       name: "",
       value: protoscript.AnyJSON.initialize(),
+      ...msg,
     };
   },
 
diff --git a/packages/protoscript/src/runtime/well-known-types/wrappers.pb.ts b/packages/protoscript/src/runtime/well-known-types/wrappers.pb.ts
index a3dbfc0..be9993a 100644
--- a/packages/protoscript/src/runtime/well-known-types/wrappers.pb.ts
+++ b/packages/protoscript/src/runtime/well-known-types/wrappers.pb.ts
@@ -145,9 +145,10 @@ export const DoubleValue = {
   /**
    * Initializes DoubleValue with all fields set to their default value.
    */
-  initialize: function (): DoubleValue {
+  initialize: function (msg?: Partial<DoubleValue>): DoubleValue {
     return {
       value: 0,
+      ...msg,
     };
   },
 
@@ -212,9 +213,10 @@ export const FloatValue = {
   /**
    * Initializes FloatValue with all fields set to their default value.
    */
-  initialize: function (): FloatValue {
+  initialize: function (msg?: Partial<FloatValue>): FloatValue {
     return {
       value: 0,
+      ...msg,
     };
   },
 
@@ -279,9 +281,10 @@ export const Int64Value = {
   /**
    * Initializes Int64Value with all fields set to their default value.
    */
-  initialize: function (): Int64Value {
+  initialize: function (msg?: Partial<Int64Value>): Int64Value {
     return {
       value: 0n,
+      ...msg,
     };
   },
 
@@ -346,9 +349,10 @@ export const UInt64Value = {
   /**
    * Initializes UInt64Value with all fields set to their default value.
    */
-  initialize: function (): UInt64Value {
+  initialize: function (msg?: Partial<UInt64Value>): UInt64Value {
     return {
       value: 0n,
+      ...msg,
     };
   },
 
@@ -413,9 +417,10 @@ export const Int32Value = {
   /**
    * Initializes Int32Value with all fields set to their default value.
    */
-  initialize: function (): Int32Value {
+  initialize: function (msg?: Partial<Int32Value>): Int32Value {
     return {
       value: 0,
+      ...msg,
     };
   },
 
@@ -480,9 +485,10 @@ export const UInt32Value = {
   /**
    * Initializes UInt32Value with all fields set to their default value.
    */
-  initialize: function (): UInt32Value {
+  initialize: function (msg?: Partial<UInt32Value>): UInt32Value {
     return {
       value: 0,
+      ...msg,
     };
   },
 
@@ -547,9 +553,10 @@ export const BoolValue = {
   /**
    * Initializes BoolValue with all fields set to their default value.
    */
-  initialize: function (): BoolValue {
+  initialize: function (msg?: Partial<BoolValue>): BoolValue {
     return {
       value: false,
+      ...msg,
     };
   },
 
@@ -614,9 +621,10 @@ export const StringValue = {
   /**
    * Initializes StringValue with all fields set to their default value.
    */
-  initialize: function (): StringValue {
+  initialize: function (msg?: Partial<StringValue>): StringValue {
     return {
       value: "",
+      ...msg,
     };
   },
 
@@ -681,9 +689,10 @@ export const BytesValue = {
   /**
    * Initializes BytesValue with all fields set to their default value.
    */
-  initialize: function (): BytesValue {
+  initialize: function (msg?: Partial<BytesValue>): BytesValue {
     return {
       value: new Uint8Array(),
+      ...msg,
     };
   },
 
@@ -749,9 +758,10 @@ export const DoubleValueJSON = {
   /**
    * Initializes DoubleValue with all fields set to their default value.
    */
-  initialize: function (): DoubleValue {
+  initialize: function (msg?: Partial<DoubleValue>): DoubleValue {
     return {
       value: 0,
+      ...msg,
     };
   },
 
@@ -801,9 +811,10 @@ export const FloatValueJSON = {
   /**
    * Initializes FloatValue with all fields set to their default value.
    */
-  initialize: function (): FloatValue {
+  initialize: function (msg?: Partial<FloatValue>): FloatValue {
     return {
       value: 0,
+      ...msg,
     };
   },
 
@@ -853,9 +864,10 @@ export const Int64ValueJSON = {
   /**
    * Initializes Int64Value with all fields set to their default value.
    */
-  initialize: function (): Int64Value {
+  initialize: function (msg?: Partial<Int64Value>): Int64Value {
     return {
       value: 0n,
+      ...msg,
     };
   },
 
@@ -905,9 +917,10 @@ export const UInt64ValueJSON = {
   /**
    * Initializes UInt64Value with all fields set to their default value.
    */
-  initialize: function (): UInt64Value {
+  initialize: function (msg?: Partial<UInt64Value>): UInt64Value {
     return {
       value: 0n,
+      ...msg,
     };
   },
 
@@ -957,9 +970,10 @@ export const Int32ValueJSON = {
   /**
    * Initializes Int32Value with all fields set to their default value.
    */
-  initialize: function (): Int32Value {
+  initialize: function (msg?: Partial<Int32Value>): Int32Value {
     return {
       value: 0,
+      ...msg,
     };
   },
 
@@ -1009,9 +1023,10 @@ export const UInt32ValueJSON = {
   /**
    * Initializes UInt32Value with all fields set to their default value.
    */
-  initialize: function (): UInt32Value {
+  initialize: function (msg?: Partial<UInt32Value>): UInt32Value {
     return {
       value: 0,
+      ...msg,
     };
   },
 
@@ -1061,9 +1076,10 @@ export const BoolValueJSON = {
   /**
    * Initializes BoolValue with all fields set to their default value.
    */
-  initialize: function (): BoolValue {
+  initialize: function (msg?: Partial<BoolValue>): BoolValue {
     return {
       value: false,
+      ...msg,
     };
   },
 
@@ -1113,9 +1129,10 @@ export const StringValueJSON = {
   /**
    * Initializes StringValue with all fields set to their default value.
    */
-  initialize: function (): StringValue {
+  initialize: function (msg?: Partial<StringValue>): StringValue {
     return {
       value: "",
+      ...msg,
     };
   },
 
@@ -1165,9 +1182,10 @@ export const BytesValueJSON = {
   /**
    * Initializes BytesValue with all fields set to their default value.
    */
-  initialize: function (): BytesValue {
+  initialize: function (msg?: Partial<BytesValue>): BytesValue {
     return {
       value: new Uint8Array(),
+      ...msg,
     };
   },