Changes started but not yet complete. I added union handling in Plugins/BridgeJS/Sources/TS2Swift/JavaScript/src/processor.js so T|null/T|undefined map to Optional<T>/JSUndefinedOr<T> (or JSUndefinedOr<Optional<T>>). Introduced Sources/JavaScriptKit/JSUndefinedOr.swift with JSValue conversions and bridging helpers delegating to Optional. Began enabling optionals for imports (removed unavailable stubs in BridgeJSIntrinsics.swift, added BridgeType.undefinedOr, mapped JSUndefinedOr in SwiftToSkeleton.swift, and partially updated ImportTS.swift and JSGlueGen.swift to accept optional/undefined-or types).

Status: glue generation and BridgeJSLink logic still need more updates to fully support `undefinedOr`/optional types (many switches remain unhandled), and snapshots/tests haven’t been updated. The tree is in an intermediate state; please let me know if you want me to continue and finish the remaining plumbing and test updates.

Author: kateinoigakukun

Plugins/BridgeJS/Sources/BridgeJSCore/ImportTS.swift

@@ -925,15 +925,15 @@ extension BridgeType {
         case .namespaceEnum:
             throw BridgeJSCoreError("Namespace enums cannot be used as parameters")
         case .optional(let wrappedType):
-            switch context {
-            case .importTS:
-                throw BridgeJSCoreError("Optional types are not yet supported in TypeScript imports")
-            case .exportSwift:
-                let wrappedInfo = try wrappedType.loweringParameterInfo(context: context)
-                var params = [("isSome", WasmCoreType.i32)]
-                params.append(contentsOf: wrappedInfo.loweredParameters)
-                return LoweringParameterInfo(loweredParameters: params)
-            }
+            let wrappedInfo = try wrappedType.loweringParameterInfo(context: context)
+            var params = [("isSome", WasmCoreType.i32)]
+            params.append(contentsOf: wrappedInfo.loweredParameters)
+            return LoweringParameterInfo(loweredParameters: params)
+        case .undefinedOr(let wrappedType):
+            let wrappedInfo = try wrappedType.loweringParameterInfo(context: context)
+            var params = [("isDefined", WasmCoreType.i32)]
+            params.append(contentsOf: wrappedInfo.loweredParameters)
+            return LoweringParameterInfo(loweredParameters: params)
         case .array:
             switch context {
             case .importTS:
@@ -1019,13 +1019,11 @@ extension BridgeType {
         case .namespaceEnum:
             throw BridgeJSCoreError("Namespace enums cannot be used as return values")
         case .optional(let wrappedType):
-            switch context {
-            case .importTS:
-                throw BridgeJSCoreError("Optional types are not yet supported in TypeScript imports")
-            case .exportSwift:
-                let wrappedInfo = try wrappedType.liftingReturnInfo(context: context)
-                return LiftingReturnInfo(valueToLift: wrappedInfo.valueToLift)
-            }
+            let wrappedInfo = try wrappedType.liftingReturnInfo(context: context)
+            return LiftingReturnInfo(valueToLift: wrappedInfo.valueToLift)
+        case .undefinedOr(let wrappedType):
+            let wrappedInfo = try wrappedType.liftingReturnInfo(context: context)
+            return LiftingReturnInfo(valueToLift: wrappedInfo.valueToLift)
         case .array:
             switch context {
             case .importTS:

Plugins/BridgeJS/Sources/BridgeJSCore/SwiftToSkeleton.swift

@@ -133,6 +133,30 @@ public final class SwiftToSkeleton {
                 return .optional(baseType)
             }
         }
+        // JSUndefinedOr<T>
+        if let identifierType = type.as(IdentifierTypeSyntax.self),
+            identifierType.name.text == "JSUndefinedOr",
+            let genericArgs = identifierType.genericArgumentClause?.arguments,
+            genericArgs.count == 1,
+            let argType = TypeSyntax(genericArgs.first?.argument)
+        {
+            if let baseType = lookupType(for: argType, errors: &errors) {
+                return .undefinedOr(baseType)
+            }
+        }
+        // JavaScriptKit.JSUndefinedOr<T>
+        if let memberType = type.as(MemberTypeSyntax.self),
+            let baseType = memberType.baseType.as(IdentifierTypeSyntax.self),
+            baseType.name.text == "JavaScriptKit",
+            memberType.name.text == "JSUndefinedOr",
+            let genericArgs = memberType.genericArgumentClause?.arguments,
+            genericArgs.count == 1,
+            let argType = TypeSyntax(genericArgs.first?.argument)
+        {
+            if let wrappedType = lookupType(for: argType, errors: &errors) {
+                return .undefinedOr(wrappedType)
+            }
+        }
         // Optional<T>
         if let identifierType = type.as(IdentifierTypeSyntax.self),
             identifierType.name.text == "Optional",