feat: allow undefined when property optional for exactOptionalPropertyTypes

Author: kuhe

smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/StructuredMemberWriter.java

@@ -106,6 +106,9 @@ void writeMembers(TypeScriptWriter writer, Shape shape) {
             String optionalSuffix = shape.isUnionShape() || !isRequiredMember(member) ? "?" : "";
             String typeSuffix = requiredMemberMode == RequiredMemberMode.NULLABLE
                     && isRequiredMember(member) ? " | undefined" : "";
+            if (optionalSuffix.equals("?")) {
+                typeSuffix = " | undefined"; // support exactOptionalPropertyTypes.
+            }
             writer.write("${L}${L}${L}: ${T}${L};", memberPrefix, memberName, optionalSuffix,
                     symbolProvider.toSymbol(member), typeSuffix);
 

smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/TypeScriptSettings.java

@@ -558,7 +558,7 @@ public enum RequiredMemberMode {
         NULLABLE("nullable"),
 
         /**
-         * This will dissallow members marked as {@link RequiredTrait} to be {@code undefined}.
+         * This will disallow members marked as {@link RequiredTrait} to be {@code undefined}.
          * Use this mode with CAUTION because it comes with certain risks. When a server drops
          * {@link RequiredTrait} from an output shape (and it is replaced with {@link DefaultTrait}
          * as defined by the spec), if the server does not always serialize a value,

smithy-typescript-codegen/src/test/java/software/amazon/smithy/typescript/codegen/StructureGeneratorTest.java

@@ -81,7 +81,7 @@ public void properlyGeneratesOptionalNonMessageMemberOfException() {
                 "export class Err extends __BaseException {\n"
                         + "  readonly name: \"Err\" = \"Err\";\n"
                         + "  readonly $fault: \"client\" = \"client\";\n"
-                        + "  foo?: string;\n"
+                        + "  foo?: string | undefined;\n"
                         + "  /**\n"
                         + "   * @internal\n"
                         + "   */\n"
@@ -525,7 +525,7 @@ public void generatesNonErrorStructures() {
         String output = writer.toString();
 
         assertThat(output, containsString("export interface Bar {"));
-        assertThat(output, containsString("foo?: string;"));
+        assertThat(output, containsString("foo?: string | undefined;"));
     }
 
     private StructureShape createNonErrorStructure() {