diff --git a/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache b/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
index 1a9eeaf4cb2..d29baae425a 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
@@ -333,10 +333,11 @@ export function exists(json: any, key: string) {
 
 {{^withoutRuntimeChecks}}
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 {{/withoutRuntimeChecks}}
 
diff --git a/samples/client/others/typescript-fetch/self-import-issue/runtime.ts b/samples/client/others/typescript-fetch/self-import-issue/runtime.ts
index 955ce4eb4bc..c3175f3bac2 100644
--- a/samples/client/others/typescript-fetch/self-import-issue/runtime.ts
+++ b/samples/client/others/typescript-fetch/self-import-issue/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/runtime.ts b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/runtime.ts
index 67d1bfe706d..c9b10a3dc88 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/runtime.ts b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/runtime.ts
index 67d1bfe706d..c9b10a3dc88 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts
index 8f32bcbfe8c..6d3d965df11 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {
diff --git a/samples/client/petstore/typescript-fetch/builds/default/runtime.ts b/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
index 373faeefbab..3f125c3b060 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {
diff --git a/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts b/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts
index 38159129a89..8095de29bc3 100644
--- a/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts
index 373faeefbab..3f125c3b060 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts
index 373faeefbab..3f125c3b060 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {
diff --git a/samples/client/petstore/typescript-fetch/builds/oneOf/runtime.ts b/samples/client/petstore/typescript-fetch/builds/oneOf/runtime.ts
index 01b09b35811..c807c6354f8 100644
--- a/samples/client/petstore/typescript-fetch/builds/oneOf/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/oneOf/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts
index 373faeefbab..3f125c3b060 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts
index 373faeefbab..3f125c3b060 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/runtime.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/runtime.ts
index 8f32bcbfe8c..6d3d965df11 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {
diff --git a/samples/client/petstore/typescript-fetch/builds/validation-attributes/runtime.ts b/samples/client/petstore/typescript-fetch/builds/validation-attributes/runtime.ts
index 373faeefbab..3f125c3b060 100644
--- a/samples/client/petstore/typescript-fetch/builds/validation-attributes/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/validation-attributes/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
index 373faeefbab..3f125c3b060 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts
index 373faeefbab..3f125c3b060 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {
diff --git a/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts b/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts
index 38159129a89..8095de29bc3 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts
@@ -343,10 +343,11 @@ export function exists(json: any, key: string) {
 }
 
 export function mapValues(data: any, fn: (item: any) => any) {
-  return Object.keys(data).reduce(
-    (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
-    {}
-  );
+    const result: { [key: string]: any } = {};
+    for (const key of Object.keys(data)) {
+        result[key] = fn(data[key]);
+    }
+    return result;
 }
 
 export function canConsumeForm(consumes: Consume[]): boolean {