[typescript-fetch] Fix ToJSON for non-descriminator oneOf constructs (#12513)

* [typescript-fetch] Fix ToJSON for non-descriminator oneOf constructs

* [typescript-fetch] Update samples for oneOf fix

modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache

@@ -20,6 +20,20 @@ import {
 {{/discriminator}}
 {{>modelGenericInterfaces}}
 
+/**
+ * Check if a given object implements the {{classname}} interface.
+ */
+export function instanceOf{{classname}}(value: object): boolean {
+    let isInstance = true;
+    {{#vars}}
+    {{#required}}
+    isInstance = isInstance && "{{name}}" in value;
+    {{/required}}
+    {{/vars}}
+
+    return isInstance;
+}
+
 export function {{classname}}FromJSON(json: any): {{classname}} {
     return {{classname}}FromJSONTyped(json, false);
 }

modules/openapi-generator/src/main/resources/typescript-fetch/modelOneOf.mustache

@@ -2,6 +2,7 @@
 {{#oneOf}}
 import {
     {{{.}}},
+    instanceOf{{{.}}},
     {{{.}}}FromJSON,
     {{{.}}}FromJSONTyped,
     {{{.}}}ToJSON,
@@ -51,7 +52,14 @@ export function {{classname}}ToJSON(value?: {{classname}} | null): any {
             throw new Error(`No variant of {{classname}} exists with '{{discriminator.propertyName}}=${value['{{discriminator.propertyName}}']}'`);
     }
 {{/discriminator}}
+
 {{^discriminator}}
-    return { {{#oneOf}}...{{{.}}}ToJSON(value){{^-last}}, {{/-last}}{{/oneOf}} };
+    {{#oneOf}}
+    if (instanceOf{{{.}}}(value)) {
+        return {{{.}}}ToJSON(value as {{{.}}});
+    }
+    {{/oneOf}}
+
+    return {};
 {{/discriminator}}
 }