- fixes missing return type bug in TS

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Fixed a bug where scalar error mappings would be generated even though it's not supported by the http request adapter. [#4018](https://github.com/microsoft/kiota/issues/4018)
 - Switched to proxy generation for TypeScript, leading to about ~44% bundle sizes reduction. [#3642](https://github.com/microsoft/kiota/issues/3642)
+- Fixed a bug where TypeScript models factory methods would be missing return types.
 
 ## [1.10.1] - 2024-01-12
 

src/Kiota.Builder/CodeDOM/CodeParameter.cs

@@ -52,6 +52,10 @@ public enum CodeParameterKind
     /// The content type of the request body when it couldn't be inferred from the description.
     /// </summary>
     RequestBodyContentType,
+    /// <summary>
+    /// When the deserialization method is replaced as a function, this is the parameter representing instance we're deserializing into.
+    /// </summary>
+    DeserializationTarget,
 }
 
 public class CodeParameter : CodeTerminalWithKind<CodeParameterKind>, ICloneable, IDocumentedElement, IDeprecableElement

src/Kiota.Builder/Refiners/TypeScriptRefiner.cs

@@ -736,7 +736,8 @@ private static void AddInterfaceParamToSerializer(CodeInterface modelInterface,
         {
             Name = ReturnFinalInterfaceName(modelInterface.Name), // remove the interface suffix
             DefaultValue = "{}",
-            Type = new CodeType { Name = ReturnFinalInterfaceName(modelInterface.Name), TypeDefinition = modelInterface }
+            Type = new CodeType { Name = ReturnFinalInterfaceName(modelInterface.Name), TypeDefinition = modelInterface },
+            Kind = CodeParameterKind.DeserializationTarget,
         });
 
         if (modelInterface.Parent is not null)

src/Kiota.Builder/Writers/TypeScript/CodeFunctionWriter.cs

@@ -26,7 +26,9 @@ public override void WriteCodeElement(CodeFunction codeElement, LanguageWriter w
 
         var codeMethod = codeElement.OriginalLocalMethod;
 
-        var returnType = codeMethod.Kind != CodeMethodKind.Factory ? conventions.GetTypeString(codeMethod.ReturnType, codeElement) : string.Empty;
+        var returnType = codeMethod.Kind is CodeMethodKind.Factory ?
+            "((instance?: Parsable) => Record<string, (node: ParseNode) => void>)" :
+            conventions.GetTypeString(codeMethod.ReturnType, codeElement);
         var isVoid = "void".EqualsIgnoreCase(returnType);
         CodeMethodWriter.WriteMethodDocumentationInternal(codeElement.OriginalLocalMethod, writer, isVoid, conventions);
         CodeMethodWriter.WriteMethodPrototypeInternal(codeElement.OriginalLocalMethod, writer, returnType, isVoid, conventions, true);

src/Kiota.Builder/Writers/TypeScript/TypeScriptConventionService.cs

@@ -56,8 +56,18 @@ public override string GetParameterSignature(CodeParameter parameter, CodeElemen
     {
         ArgumentNullException.ThrowIfNull(parameter);
         var paramType = GetTypeString(parameter.Type, targetElement);
-        var defaultValueSuffix = string.IsNullOrEmpty(parameter.DefaultValue) ? string.Empty : $" = {parameter.DefaultValue} as {paramType}";
-        return $"{parameter.Name.ToFirstCharacterLowerCase()}{(parameter.Optional && parameter.Type.IsNullable ? "?" : string.Empty)}: {paramType}{(parameter.Type.IsNullable ? " | undefined" : string.Empty)}{defaultValueSuffix}";
+        var defaultValueSuffix = (string.IsNullOrEmpty(parameter.DefaultValue), parameter.Kind) switch
+        {
+            (false, CodeParameterKind.DeserializationTarget) => $" = {parameter.DefaultValue}",
+            (false, _) => $" = {parameter.DefaultValue} as {paramType}",
+            (true, _) => string.Empty,
+        };
+        var (partialPrefix, partialSuffix) = parameter.Kind switch
+        {
+            CodeParameterKind.DeserializationTarget => ("Partial<", ">"),
+            _ => (string.Empty, string.Empty),
+        };
+        return $"{parameter.Name.ToFirstCharacterLowerCase()}{(parameter.Optional && parameter.Type.IsNullable ? "?" : string.Empty)}: {partialPrefix}{paramType}{partialSuffix}{(parameter.Type.IsNullable ? " | undefined" : string.Empty)}{defaultValueSuffix}";
     }
     public override string GetTypeString(CodeTypeBase code, CodeElement targetElement, bool includeCollectionInformation = true, LanguageWriter? writer = null)
     {